1997 - Department of Astronomy - University of Maryland

233

University of MarylandDepartment of Astronomy

College Park, Maryland 20742

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1. INTRODUCTIONThis report covers astronomical activities primarily with

Maryland’s Department of Astronomy but also includsome astronomical work carried out in other departmesuch as the Department of Physics. The period coverefrom 1 October 1996 to 30 September 1997.

2. PERSONNELThe continuing personnel in the Department of A

tronomy during the period were Professors M. Levent~Chair!, M. F. A’Hearn, R. A. Bell, J. P. Harrington, M. RKundu, K. Papadopoulos, W. K. Rose, V. L. Trimble,Vogel, and A. S. Wilson; Emeritus Professors W. C. Ericson, F. J. Kerr, and D. G. Wentzel; Adjunct ProfessorsHauser and S. Holt; Associate Professors A. Harris, L.Mundy, and J. M. Stone; Assistant Professors D. HamiltE. Ostriker, J. Wang, and S. Veilleux; Associate ReseaScientists K. Arnaud, M. Aschwanden, C. C. Goodrich,Lopez, L. McFadden, G. Milikh, E. Schmahl, S. Sharma aS. White; Assistant Research Scientists S. BalachandranT. Golla; Research Associates W. Chen, L. Cheng, F. Fbeiner, D. Chornay, O. Colombo, K. Fast, K. Gendreau,Grayzeck, M. Houdashelt, U. Hwang, N. Lame, J. Lee,Lisse, T. Livengood, M. Lowenstein, G. Madjeski, J. Mogan, F. Porter, A. Raugh, I. Richardson, K. Roettiger,Safier, C. Scharf, P. Teuben and D. Wellnitz; InstructorsDeming and D. Theison; and Associate Director J.Trasco. M. Bautista, S. Dinyes, P. Ferruit, K. Issak, T. Lengood, C. Mundell, N. Pereyra, K. Rauch, J. Staguhn,Stark, and M. Wolfire were appointed as Research Assates. P. Chatuverdi, H. Falcke, N. Gopalswamy, F. LemoE. Pavlis, F. Robinson, and R. Schulz completed short tappointments and left the department. Ph.D. degrees wawarded to J. Braatz, E. Colbert, M. Regan, and M. Tholey. An M.S. degree was awarded to L. Woodney.

V. Trimble was elected vice president of the AmericAstronomical Society for the term 1997-2000. She will cotinue to serve as Vice President of the International Asnomical Union for the same term.

3. MEETINGSThe series of Washington Area Astronomers meetings

tiated in September 1981 has continued. The Fall 1996 ming was held at the the NASA-Goddard Space Flight Cenand the Spring 1997 meeting was held at the UniversityMaryland. Typical attendance has been 100 personsmeeting. The Maryland-Goddard Astrophysics series whstarted in Fall 1990 has continued. The most recent mee‘‘Accretion Processes in Astrophysical Systems’’ was heldthe University of Maryland in October 1997. Total attedance was approximately 200 persons.

The second Maryland-Goddard Interaction Day with tobjective of promoting further interaction between the

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searchers at UMCP and GSFC was held on February1997. This year’s theme was Sun-Earth Connection.

A new program within the University’s College ParScholars Program, a residential, living- learning programacademically talented students, was initiated this year frthe Astronomy Department and the College of CompuMathematical and Physical Sciences. The program in Sence, Discovery and the Universe, is headed by facultyrector, Lucy McFadden. Approximately 50 students primrily interested in the physical sciences are currently admitin this program.

4. FACILITIES AND INSTRUMENTATION

4.1 Laboratory for Millimeter-Wave Astronomy

The Laboratory for Millimeter-wave Astronomy~LMA !is the organization set up by the University of Marylandmanage its participation in the Berkeley-Illinois-MarylanAssociation~BIMA ! project, which has led to the construction of a ten radio-dish millimeter array located in Hat CreeCalifornia. The lab is part of the Astronomy Department ahas associated with it five faculty members, one reseascientist, three scientific staff, four postdoctoral fellows, afive graduate students. The faculty are S. Vogel~director!,W. Erickson ~emeritus!, A. Harris, M. Kundu, and L.Mundy. The Associate Research Scientist is S. White. Pdoctoral fellows include P. Safier, K. Isaak, M. Choi, andStaguhn. The scientific staff includes M. Pound, J. Morgn,and P. Teuben. Graduate students doing Ph.D. degreewith the array include S. Donaldson, C. Lee, L. Looney,Sheth, and L. Woodney. M. Regan and M. Thornley copleted their Ph.D. theses and assumed a Hubble Fellowat DTM and a Humboldt Fellowship at Garching, respetively.

The scientific work done with the BIMA array is outlinein the relevant sections of this report. During the past yethe array was brought to 10 antennas. New stations wadded to extend the array to provide maximum baseline1.3 km and a resolution at 3 mm of 0.49, the longest baselines available with any connected-element millimeter arrMundy, Looney, and Erickson from Maryland designed abuilt the optical fiber links to carry local oscillator signalsand return the astronomical signals from the long-baseantenna pads. In addition, optical fiber fixed and variadelay systems were built to enable real time correlationastronomical signals.

Maryland is jointly responsible with Illinois for the calibration, analysis, and image reconstruction software forBIMA consortium. Morgan and Teuben continued the joBIMA development of the MIRIAD data reduction packagThe Maryland group focuses on antenna-based calibraroutines for use with the newly expanded Hat Creek 1element array; reduction and analysis of long-baseline inferometric data including atmospheric phase correcti

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234 ANNUAL REPORT

rapid time integration routines for solar flares and eclipsholography techniques for the new antennas; observing sware; analysis applications in the UV-plane; the WIP intactive graphics tool; and various X-window oriented appcations.

The LMA has continued to develop its local World WidWeb ~WWW! server to provide world wide access to infomation related to the BIMA project, observing at the HCreek telescope, the MIRIAD software package, andUniversity of Maryland Astronomy Department.

The LMA has also continued its involvement with thAIPS11 software development, working on a filler foBIMA uv data.

Maryland is designated as the East Coast observingfor the array. Thirty percent of the observing time on tarray is generally available to outside users~outside of thetime designated for system development and tests!.

Expansion of the BIMA array baselines to maximulengths of 1.4 km was completed in the Winter of 1996-9The BIMA array is now the highest resolution millimetewavelength array in the world. L. Mundy and L. Looneplayed a principle role in the expansion, designing and buing the optical fiber communication links and variable desystem. The system was operated in December and Janyielding excellent results; one-half arcsecond resolutionages of more than a dozen young stellar systems weretained. As a next step in expanding the array, anothertenna station is being built this year at a distance of 1.2from the center of the main array. When completed,maximum baseline on the array will be nearly 2 km andspatial resolution at 2.7mm wavelength will be appromately 0.3 arcseconds.

4.2 Advanced Visualization Laboratory

To meet the widely perceived need for data visualizatsupport on campus, the Department of Astronomy andComputer Science Center jointly established the AdvanVisualization Laboratory~AVL !, under the direction of CGoodrich ~Astronomy!. This laboratory serves as the focpoint for visualization expertise on campus providing infomation and demonstrations of state of the art visualizasoftware and hardware. Their goal is to provide graphsupport and innovation focusing on the types of compugenerally available to researchers and students on camThe functions of the laboratory are at three levels. Atmost basic level, the AVL provides color input/output facities on a fee-for-materials and nominal service charge baincluding paper and transparency color printing, 35mm firecording, color scanning, and SVHS/VHS videotape pduction and editing. At a higher level, the AVL providefaculty and students the opportunity for hands-on evaluaof a variety of visualization software on a range of computin a friendly environment. Basic assistance in the use ofsualization software and devices is available from the laratory staff, and classes in scientific visualization are undevelopment. Finally, the AVL actively pursues joiprojects with researchers to develop custom visualizatools and works with hardware and software vendorsevaluate, test, and enhance available commercial soluti

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The AVL has current research collaborations in the spsciences, mathematics, and fishery management.

4.3 Planetary Data System

The Small Bodies Node~SBN! of the Planetary Data System ~PDS! is being operated in the department underdirection of M. F. A’Hearn and E. Grayzeck. A. Raugh anD. Winterfeld provide programming support. Sub-nodesoperated at the U. Hawaii, U. Arizona, Planetary ScienInstitute, and Konkoly Observatory~the European Sub-node!in Budapest, Hungary. The Sub-nodes archive relevantfor comets, asteroids, and interplanetary dust; they alsotribute data once it has been reviewed by the PDS. Thmajor sets of data have now been reviewed and have bingested into the PDS: the Comet Halley CD-ROM Archiof 24 disks produced by the International Halley Watch wSBN assistance, data sets of derived asteroid propertiewell as fundamental data, and IRAS data determined touseful for studying interplanetary dust. Most data are avable through the SBN computer system interface that allosimple browsing and data requests; SBN also has a hpage on WWW. Sub-nodes are actively involved in achiving spacecraft data from various missions. These dsets include data from the group of spacecraft that encotered comet Halley in 1986 and the Giotto Extended Missto comet Grigg-Skjellerup in 1992, the Galileo experimenthat collected data at the asteroid Gaspra encounter, anterplanetary dust measurements from Ulysses and GalWork also continues on certain ground-based data for asoids which supersedes the Asteroids II database and preing IRAS data in forms suitable for investigating solar sytem sources. To support the customized GEM archivecomet Grigg-Skjellerup, SBN has developed techniquesCD Write Once production using networked hardware. TPDS-SBN has played a major part in the campaign to stthe impact of comet Shoemaker-Levy 9 into Jupiter. Its potal role as communications hub during impact week ctributed to the success of observing campaigns literaaround the globe. The node continues to operate an etronic bulletin board which serves as an official point of cotact for release of announcements by funding agenciescilities, and scientists. PDS has also produced a CD-Rarchive of data acquired during this event.

5. RESEARCH

5.1 Extragalactic Astronomy

G. Welch, L. Sage, and G. Mitchell continue to study tmolecular gas content of the dwarf companions of M3They have found molecular gas in two~NGC 185 and NGC205!, but there is apparently no gas in either NGC 147M32, which are comparable in overall mass and morpholocal type. In addition, they have begun an unbiased searchmolecular and atomic gas in SO galaxies, using a distanlimited sample.

Results from a multi-wavelength study of the prototypicbarred spiral NGC 5383 by K. Sheth, M. Regan, S. VogelTeuben and J. Stone were presented at the winter AAS ming. An analysis of the gas kinematics using the CO d

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235UNIVERSITY OF MARYLAND

obtained with the BIMA interferometer corroborated the pdictions of hydrodynamic simulations by Regan and by Piet al., in particular the spray from the dust lane - ILR contapoint. Comparison of the CO distribution with further nmerical modeling shows the need for the inclusion of sformation in any realistic model of gas flow in a barred ptential. Additionally, a remarkable correlation, better thhas been observed in any other barred galaxy, is obsebetween the CO distribution and dust extinction morpholowhich may be a consequence of the lower star formationin NGC 5383. K. Sheth and M. Regan are also studyingemission in several barred galaxies to determine the kinemics of gas flow in barred potentials.

CO emission from giant molecular clouds in the northespiral arm of the Andromeda~M31! galaxy was observedwith the BIMA array by K. Sheth, S. Vogel and C. Wilso~McMaster!. Contrary to expectations, the region did ncontain a single CO-bright GMC. The GMCs which weidentifiable were far fewer than expected with low flux~barely above the noise! and small linewidths. These resulare quite contradictory to those previously publishedM31 GMCs and in stark contrast to the GMC populationthe other nearby spiral galaxy M33. A more sensitive survof a larger area of the spiral arm in M31 is underway.

S. Vogel, S. Veilleux, and R. Weymann~Carnegie Obser-vatories! are continuing their collaboration to detect ameasure the metagalactic ionizing flux at zero redshift ustheir newly constructed reimager with an FP system atLas Campanas Dupont 2.5 meter telescope.

M. Regan has completed an infrared survey of a samof Seyfert and control galaxies with J. Mulchaey~Carnegie!and A. Kundu. The goal of the survey is to determiwhether the Seyfert galaxies are any more ‘‘barred’’ thancontrol galaxies. The survey used telescopes at Kitt PLas Campanas, and Palomar observatories.

M. Thornley and L. Mundy are continuing to collaboraon a series of papers on the gas properties and dynamiflocculent galaxies. Two papers appeared this year, oneNGC 5055 and the other on NGC 4414~Thornley andMundy 1997!. The central results of these papers are aslows. Many flocculent galaxies show organized structuresscales larger than expected for stochastic star formationthe case of NGC 5055, there is kinematic evidence that thstructure are organized by weak spiral density waves. In geral, we argue that flocculent galaxies should be vieweddisk galaxies with weak density waves and they represenweak end density wave activity with grand design spiralsthe strong end of the range. The interstellar medium in flculent galaxies also shows strong similarities to other tyof spiral galaxies. In particular, giant molecular clouds agiant molecular cloud complexes are present in the diskflocculent galaxies.

In collaboration with T. Storchi-Bergmann~UFRGS, Bra-zil!, L. Binette~ESO! and J. S. Mulchaey~CIW!, Wilson hasobtained high signal to noise optical spectra of extendhigh excitation nebulosities reaching several kiloparsfrom the nuclei of five Seyfert and narrow-line radio galaies. It is found that the conventional description of thephotoionized gases - radiation-bounded~i.e. optically thick!

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clouds with a range of ionization parameter~U! - is unable toaccount for the line ratios seen. The authors propose insthat two populations of ionized clouds contribute to the liemission: a matter-bounded component responsible for mof the He II and high excitation forbidden line emission aa radiation-bounded component emitting low-to-intermediexcitation lines. component emitting low-to-intermediate ecitation lines. A new sequence of photoionization calcutions is thereby obtained~using the code MAPPINGS! byvarying AR/M , which is defined as the solid angle ratio ocupied by the radiation-bounded component relative tomatter-bounded component. In various line ratio diagrathe AR/M–sequence is compared to the traditional sincomponent U–sequence. The main success of the modthat it provides a natural explanation to the newly foucorrelations between the He12/Hb ratio and the@OII#/@NeV#,@OII#/@OIII # and@SII#/Ha ratios. The U–sequence cannot acount for these. Furthermore, the AR/M sequence producemuch stronger high excitation lines of@NeV# and CIVll1549 than the U–sequence, in accordance with obsetions.

Wilson, Storchi-Bergmann and Binette have also obtainmeasurements of the electron temperature at about a dlocations in the extended emission-line regions of active gaxies. Temperatures~T[OIII ] and T[NII ] ) have been determined from both the I~@OIII #l4363!fi~@OIII #l5007! andI~@NII #l5755!/I@NII #l6583! ratios. There is a strong trenfor T[OIII ] to be higher than T[NII ] , with the difference typi-cally being' 5,000K. The measured values of T[OIII ] andthe differences between T[OIII ] and T[NII ] are very similar tothose found in Galactic planetary nebulae. It is argued tthe dominant form of energy input to the clouds is photoioization, but detailed modelling indicates that the temperatdifference is too large to be accounted for in terms of phtoionization of radiation-bounded clouds. It is proposedstead that both matter- and radiation-bounded cloudspresent in the extended emission-line regions, with mosthe@OIII # emission originating from a hot zone in the mattebounded clouds and essentially all of the@NII # from theradiation–bounded clouds.

In collaboration with J. A. Acosta-Pulido~IAC!, B. Vila-Vilaro ~Nobeyama Radio Observatory!, I. Perez-Fournon~IAC! and Z. I. Tsvetanov~JHU!, Wilson has perfomed adetailed spectral study of the extended emission-line regof the Seyfert 2 galaxy NGC 5252. The line ratios favorlarger fraction of matter-bounded~optically thin! clouds inthe extended gas than in the nucleus or an additional extion mechanism in the nucleus, such as interaction withradio jets. Another result favoring a contiribution frommatter-bounded clouds in the extended gas is a correlabetween the excitation of the gas and the emission-brightness. A broad Ha component~FWHM ' 2,500 kms21! is detected in the nucleus. The results of the study sgest that NGC 5252 contains a partially hidden broad-lregion and a more luminous nucleus than is directly oserved.

C. Simpson~JPL!, J. S. Mulchaey~CIW!, Wilson, M. J.Ward ~U. Leicester! and A. Alonso-Herrero~Madrid! havereported an optical emission-line and near infrared c

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236 ANNUAL REPORT

tinuum study of the Seyfert 2 galaxy Mkn 348. The opticimages reveal a well-defined region of highly ionized galigned approximately north-south~i.e. along the radio axis!possibly in the form of a bi-cone. In addition, there is a rlinear structure in the infrared continuum, oriented perpdicular to this axis and approximately 1 kpc in extent. Thlatter structure may be emission from hot dust in the ouparts of the ‘‘dusty torus’’ postulated in schemes to unify ttwo types of Seyfert galaxy.

J. A. Morse~CASA!, J. C. Raymond~CFA! and Wilsonhave investigated the viability of fast, photoionizing shocas an ionization mechanism in active galaxies. They dischow photoionizing shocks may help to resolve the UV phton deficit problem in some objects because a significfraction of the ionizing radiation is emitted as line emissiin the EUV. Also discussed are the implications of tstrengths of collisionally excited UV lines, the weaknessCa II lines, and the high electron temperatures derived frthe @O III #(l49591l5007)/l4363 ratio for the ionizationmechanism.

T. M. Heckman~JHU!, R. Gonzalez-Delgado~STScI!, C.Leitherer~STScI!, G. R. Meurer~JHU!, J. H. Krolik ~JHU!A. Koratkar ~STScI!, A. Kinney ~STScI! and Wilson havepresented and discussed HST images and a UV specplus new ground-based near-UV through near-IR spectrthe luminous type 2 Seyfert galaxy Mkn 477. The data pvide direct evidence that the observed UV through nearcontinuum in the nucleus of this galaxy is dominated by ligfrom a compact~few hundred pc!, dusty starburst. For example, in the space-UV, strong NVl1240 and SiIVl1400stellar wind lines and other weak photospheric lines fromstars are detected. Detailed comparison of the data to moimplies that the starburst has an age of 6 Myr, was of shduration, has solar or higher metallicity and a bolometluminosity of ' 331010 to 1011 L( . The starburst musmake a significant contribution to the overall energeticsMkn 477, even though optical spectropolarimetry demostrates that a powerful ‘hidden’ type 1 Seyfert nucleus is apresent.

Recent measurements of the Circinus galaxy with ISOMoorwoodet al. ~1996! have confirmed the extremely higexcitation of the emission-line spectrum in this active galaL. Binette, Wilson, A. Raga~UNAM ! and T Storchi-Bergmann have interpreted these results with the help ofphotoionization calculations in which all the zones of hiexcitation coexist within the internal structure of smmatter-bounded clouds. A strong UV bump is not requiredexplain the line ratios in this picture. The calculations cosider the effects of radiation pressure exerted by photoetric absorption which become very important at the high ioization parameters required by the line ratios. It is propothat the radiation pressure generates a strong density grawithin the photoionized structure and is responsible forcelerating the matter-bounded gas, explaining the systemblueshift observed for the coronal lines.

G. A. Bower ~NOAO!, T. M. Heckman~JHU!, Wilsonand D. O. Richstone~U. Michigan! have obtained opticaimages of the nucleus of the nearby radio galaxy M84~NGC437453C272.1! with the Wide Field/Planetary Camera

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~WFPC2! aboard the Hubble Space Telescope~HST!. Thesehigh-resolution images reveal that the Ha1@N II # emissionhas three components, namely a nuclear gas disk, an ‘iontion cone’, and outer filaments. The major axis of the gdisk aligns with that of the dust lanes on scales of 50 pc. Tradio jet and ‘ionization cone’ are found to align approxmately perpendicular to the gas disk.

Wilson, in collaboration with Storchi-Bergmann, MEracleous~UCB!, M. T. Ruiz ~U. Chile!, M. Livio ~STScI!and A. V. Filippenko~UCB!, has continued a study of thevolution of the broad, double-peaked Balmer lines innucleus of the LINER galaxy NGC 1097. When originaldiscovered in 1991, the red peak of Ha was stronger than theblue, while by 1994 the Ha profile had become almost symmetric. A new spectrum, taken in 1996, shows that the pfile of the Balmer lines is now such that the blue peakstronger than the red, opposite to the asymmetry observe1991. Various models are considered for the observed beior, all assuming the emission lines originate in an accretdisk. A model involving a precessing, planar, elliptical acretion ring provides an acceptable fit to the line profiles. Walso consider the possibility that the line profile evolutiresults from a precessing warp in the disk, induced by irdiation from the center, and show that the expected rangradii and precession time scales are consistent with theservations.

Stone and postdoctoral research associate K. Roettigecollaboration with R. Mushotzky~GSFC!, have studied thehydrodynamics of an off-axis merger of two clusters of gaxies using numerical simulations. Aside from providing isight into the merger process itself, the simulations also hbeen applied to accounting for the peculiar X-ray propertof the cluster Abell 754. The study of the merger processcontinuing by considering whether weak magnetic fields cbe amplified by the merger.

In collaboration with R. W. Goodrich~Keck! and G. J.Hill ~U. Texas!, Veilleux recently completed a search foobscured broad-line regions in Seyfert 2 galaxies. Forpurpose, high-quality infrared~J, K, and L bands! spectra of33 galaxies were acquired over the past four years. It wfound that a possibly large fraction~perhaps as much a25%! of the objects in the sample present a hidden broad-region. These results are consistent with the standard tmodel if the transition zone located between the opticathin ‘‘throat’’ of the torus and the optically-thick core irather extended. The dusty wind model of Konigl and Karcan also explain the data. Comparisons of the broad-linenarrow-line extinctions indicate that the BLRs are considably more obscured than the NLRs. This result agrees wthe predictions of both the torus and dusty wind modewhere the dust is located between the NLR and the BLComparisons of the columns depths towards the BLRs wthe column depths determined from X-ray data show a geral tendency for the objects with detected broad recombtion lines to have lower X-ray columns. The widths of broPab and Brg lie on the narrow end of the distribution foSeyfert 1 galaxies. The dereddened broad Hb luminositiespredicted from the broad infrared lines are similar to tbroad Hb luminosities of normal Seyfert 1s, and so are t

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ratios of the hard X-ray flux to the predicted broad Ha flux.Veilleux, D. B. Sanders~Univ. of Hawaii!, and D. C. Kim

~IPAC/Caltech! have pursued their survey of ultraluminou~log@Lir/L(# . 12! IRAS galaxies. Preliminary analysis othe optical spectra suggests that the fraction of optically csified Seyfert galaxies increases dramatically ablog @Lir/L(#512.3. Nearly half of the galaxies with log@Lir/L(# . 12.3 present Seyfert characteristics. New, recenpublished near-infrared spectra reveal signs of buried quain most Seyfert 2 galaxies. The high quasar detectionamong Seyfert 2 galaxies bring strong support to the optclassification of these objects as genuine AGN. A tendealso exists for galaxies with warm 25mm/60mm IRAS col-ors to harbor obscured broad-line regions in the near-infraand to have large Paa-to-infrared luminosity ratios. Thesresults suggest that the screen of dust in the warm Seyfegalaxies is optically thin at 2mm. Comparisons betweewarm ultraluminous infrared galaxies and optical quassuggest that an important fraction of the bolometric luminity in the warm objects is powered by the same mechanas that in optical quasars.

In an effort to evaluate the impact of nuclear activitygalaxies, Veilleux, G. N. Cecil~Univ. North Carolina!, J.Bland-Hawthorn~AAO!, and R. B. Tully~Univ. of Hawaii!are carrying out detailed Fabry-Perot studies of 20 neagalaxies. Results on the Seyfert galaxies NGC 4258 andcinus were recently published. A summary of these resalso appeared in a recent Scientific American article. Tnew data on Circinus show for the first time a complexionized filaments extending radially from the nucleus outdistances of 1 kpc. The velocity field of the filaments cofirms that they represent material expelled from the nuclor entrained in a wide-angle wind roughly aligned with tpolar axis of the galaxy. Extrapolation of these filamentssmaller radii comes to within 1 arcsec of the active galacnucleus, therefore suggesting a AGN or nuclear starburstgin to these features. The complex of radial filaments abow shocks detected in the Circinus galaxy is unique amactive galaxies. The frequency of such events is unknosince only a handful of active galaxies have been observethis sensitivity level. The event in the Circinus galaxy mrepresent a relatively common evolutionary phase inlives of gas-rich active galaxies during which the dusty ccoon surrounding the nucleus is expelled by the combiaction of jet and wind phenomena.

Wolfire is continuing his collaborations with M. Greenhouse~GSFC! and M. Laming~NRL! on the IR coronal lineemission from active galactic nuclei. In Greenhouseet al.~1977; submitted! they reported line fluxes from NGC 1068NGC 4151, and NGC 5506. In contrast to the usual assution that transitions with a high critical density show broline profiles, they found that in both NGC 1068 and NG4151 the transitions with the highest critical density havenarrowest line profiles. With Fischeret al. ~1996! Wolfireanalyzed the ISO LWS spectrum of the colliding galaxNGC 4038/39. They found significant emission arising frophotodissociated gas consistent with moderate starburstaxies.

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5.2 Galactic Astronomy

Wilson and K. W. Weiler~NRL! have described a procedure for measuring the contribution of relativistic positroto radio synchrotron radiation. The method relies on the fthat synchrotron radiation from particles of one sign~e.g.electrons! is circularly polarized by a small but measurabamount. If, on the other hand, there are equal numberrelativistic positrons and electrons, the net circular polarition is zero. The method is illustrated through high accuramapping of the circular polarization of the Crab Nebula610 MHz. No significant circular polarization was detectea very conservative limit is, 0.05%, and a more realistione , 0.03%. This upper limit on the degree of circulpolarization is comparable to or below that expected if oelectrons radiate. Various explanations are discussed, incing the possibility that relativistic positrons are present inCrab Nebula.

L. Cheng and M. Leventhal participated as major mebers in the campaign of mapping the inner Galactic 511 kpositron/electron emission led by Bill Purcell of Northwesern University. Although this annihilation signal has beknown for more than 20 years, more knowledge of the stial distribution of the Galactic Center~GC! 511 keV lineemission is needed to address its origin. The Oriented Stillation Spectrometer Experiment~OSSE! ~field of view:3.8°311.4° FWHM! on the Compton Gamma-Ray Obsevatory ~CGRO! has completed numerous observations ofGC and Galactic plane regions. These data were combwith data from earlier wide field of view instruments to mamaps of the 511 keV line distribution. The maps revealedaddition to the previously expected GC and Galactic placomponent, a surprising additional component extendingproximately 10° above the GC.

Collaborating with D. Smith~UC Berkeley!, G. Fishman~NASA-MSFC!, and J. Tueller and N. Gehrels~NASA-GSFC!, Cheng and Leventhal have finished an all-sky seafor bright, transient emission lines from 300 to 550 KeVmore than 2 years~Dec 1993 and Mar 1996! of data from theBurst And Transient Source Experiment~BATSE! on theCGRO. No convincing transient has been found in tsearch. This result, along with a similar earlier search, cthe previous discoveries of such transients into question

Cheng, Leventhal and Tueller have analyzed data frtwo 1995 Gamma-Ray Imaging Spectrometer~GRIS! bal-loon drift scan flights, in which the GC and Galactic planel5240° passed through the field-of-view of GRIS. A GC 5keV line flux that reasonably agrees with results from othinstruments has been obtained. Another 511 keV sourcefound from the analysis, though the position of this soucannot be well determined by the data. They have beenproved to use OSSE to make deep scan observations osource region to better understand the nature of this sou

Cheng, Leventhal, Tueller and J. Naya~NASA-GSFC!have been working on an atmospheric 511 keV productmodel. Atmospheric 511 keV photons are produced byannihilation of positrons, which are generated in the hadroand electro-magnetic cascades initiated by cosmic partiinjected from the top of the atmosphere. They used GEA3, the most popular Monte Carlo simulation package in h

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238 ANNUAL REPORT

energy physics. The Monte Carlo atmospheric 511 keV pduction function agrees well with the GRIS data.

M. Isenberg, D. Q. Lamb, and J. C. L. Wang have cocluded a study of how cyclotron lines form in a radiatiodriven relativistic outflow. They find that narrow features fboth the fundamental and higher harmonics are possiblecause line formation occurs primarily near the base ofoutflow. The model also predicts single-peaked emission-features which do not occur in static line forming regionThis work may be applied to Galactic halo gamma-ray bumodels and possibly to the soft-gamma repeaters.

Wolfire along with collaborators D. Hollenbach~NASA/Ames!, A.G.G.M. Tielens~NASA/Ames! and C. McKee~U.C. Berkeley! continued their work on the thermal phasesthe Galactic interstellar medium. They calculated phasegrams~thermal pressureP versus gas densityn) for Galacticradii between 3 and 18 kpc. These were constructed by sing simultaneously for the equilibrium chemical abundanand gas temperature. The calculations were carried outmanner similar to that of Wolfireet al. ~1995!, who con-structed phases diagrams for the local medium, exceptthis new work accounts for the differences in physical prameters with Galactic radii. They found that thermal pcesses allow a multiphase equilibrium in the Galactic dbetween 3 kpc and 18 kpc. The mean thermal pressure wis required ranges from aboutP/k;11,000 K cm23 at 3 kpcto P/k;1,000 K cm23 at 18 kpc.

5.3 Stellar Astronomy

Using data from the European Hipparcos Astrometric Sellite, V. Trimble and A. Kundu have completed analysisthe data pertaining to R Coronae Borealis variables and ccluded that these evolved, hydrogen poor stars must inctwo rather different populations in absolute brightness, kimatics, or both, only one of which can be thick disk or bulstars withM v524.5 to -5.5. They have partly completeconsideration of six other classes of stars included inearly-release data. Some mild surprises include the relafaintness of nova-like variable AE Aqr, the main sequennature of one supposed FK Comae~rapidly rotating giant!star, and discordance in proper motion between T Tauri sstudied from the ground and the same stars seen by Hipcos.

J. C. L. Wang and R. S. Sutherland~ANU! are studyinglow luminosity spherical accretion onto neutron starscounting for the nonlinear coupling between accretionnamics and nonequilibrium atomic processes. Using blbody ionizing spectra, they find that steady state accretiogenerally not possible owing to strong photoionization heing. They are now studying the effect of varying the ionizispectra. This work has important implications for the deteability of old neutron stars accreting from interstellar gas

With M. Isenberg and D. Q. Lamb, Wang has concludestudy of how line forming region geometry affects cyclotrresonant scattering lines. Their study shows that line shders accompanying the cyclotron fundamental are genericmoderately thick line forming regions (t l ine,;103). Ab-sence of such shoulders in observed X-ray pulsar spe

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suggests either line formation in very thick media on tstellar surface or in a radiatively supported layer of moderdepth in the magnetosphere.

Wang concludes from a study of RX J0720.4-3125,unidentified soft X-ray source with an 8.39 sec spin perithat it is consistent with an isolated old (;109 yrs! neutronstar accreting from the ISM. If so, then spontaneous mnetic field decay must have occurred unless the star waswith both a long period (.;0.5 sec! and a weak field(,;1010 G!. The required long (@107 yr! decay timescalerules out the standard picture of exponential decay ovefew million years.

Cavallo ~graduate student!, A.Sweigart~GSFC! and Bellhave studied in detail the formation of the elements C, N,Na, Mg, and Al around the hydrogen-burning shell~H shell!of globular cluster red giant stars. This is done by incorprating four stellar evolutionary sequences of different melicities with a nuclear reaction network that can be variedallow for the uncertainties in some reaction rates. Thisproach allows for the variation in the temperature and dsity around the H shell as well as the change in the strucof the star with increasing evolution.

The results show overlapping regions above the H shewhich C and then O are separately depleted. In thedepleted zone, which is closer to the H shell than thedepleted zone, Na and Al are produced from Ne andrespectively. These results are qualitatively consistent wthe observations in that they predict a decrease in C anand an increase in N, Na, and Al if non-canonical mixiwere introduced to a depth near the H shell. Since the retion rates are very temperature dependent, the variationssensitive to the environment around the H shell whchanges with evolution and is different for different metalicities. The results are consistent with the observationsthat they show that the Na and Al production increases wincreasing evolution and decreasing metallicity. The C,and O variations persist at all metallicities and evolutionastates, also in agreement with observations.

Studies of the12C/13C and16O/17O ratios show that the Cisotopes are in the equilibrium ratio near 4 above the H shas seen by several observers, and the as-yet-unknown Ois predicted to be between 50 and 150, depending onevolutionary status and metallicity of the star.

They are presently unable to model the large24Mg deple-tions seen by Shetrone~1996! in a small sample of M13giants. There appears to be no canonical method by whthis istope can be reduced, even when taking into accounlatitude afforded by the uncertainty in the NeNa- and MgAcycle reaction rates.24Mg remains abundant, in part by formation from Na created in the NeNa cycle. This is mugreater when the newer rates of El Eid and Champa~1995! are used, instead of the more widely-used, but lcertain, rates of Caughlin and Fowler~1988!.

R. D. Cannon and R. A. Stathakis~AAO!, Bell, F. W.Croke ~Crete! and J. E. Hesser~DAO! have used FOCAPmulti-object fibre optic system on the AAT to obtain specfor a sample of more than a hundred faint stars in the glolar cluster 47 Tuc~NGC 104!, from the base of the gianbranch to about a magnitude below the main sequence t

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off. The spectra cover the UV and the blue bands of CN athe G-band of CH. Variations in CN band strength areparent at all magnitude levels.

The CN and CH band strengths are anticorrelated. Fthermore, the abundance pattern appears to be bi-modasuspected previously for much smaller samples of faint sand similar to what was first found among the bright rgiants. The main sequence stars can be divided into twoproximately equal groups, one having N enhanced by ator of ;7 and C depleted by;0.6 relative to the othersimilar to what is inferred for the red giants. No significavariations are seen in calcium abundance, consistent withevidence from the narrowness of the giant branch that this very little spread in the abundance of iron and other heelements.

The CN and CH data indicate that the stellar atmosphecontain material which has been processed through the Ccycle but not through more advanced stages of nucleosynsis. In the case of the evolved red giants, this could be duthe convective dredge-up of processed material. Howethe standard models for low mass main sequence stars dhave deep convective envelopes, nor is there significCNO-cycle nucleosynthesis in their interiors. Thus the msequence C and N variations are presumably either in ssense primordial or due to a polluation mechanism. The bexplanation at the moment appears to be some sort ofenrichment process within the cluster, although no sinsimple explanation seems capable of explaining all ofobservations. Whatever the mechanism, it seems probthat these abundance data contain important clues abouformation and early evolution of globular clusters.

Harris ~McMaster!, Bell and collaborators have used thWFPC2 camera on HST to obtain photometry of the lometallicity ~@Fe/H#5-2.14!, outer-halo globular cluster NGC2419. The color-magnitude diagram in~V, V -I! reachesVlim.27.8, clearly delineating the subgiant and turnoffgion about three magnitudes of the unevolved mainquence. A differential fit of the NGC 2419 CMD to that othe similarity metal-poor ’standard’ cluster M92 shows ththey have virtually identical principal sequences and thussame age to within 1 Gyr. Previously published studiesmany other low-metallicity globular clusters throughout tMilky Way halo show that they possess this same agewithin the ;1 Gyr precision of measurement. The additiof the remote-halo object NGC 2419 to this list leads usconclude that the earliest star~or globular cluster! formationbegan at essentially the same time everywhere in the Gatic halo throughout a region now almost 200 kpc in diameThus for the metal- poorest clusters in the halo there isdetectable age gradient with Galactocentric distance.

Houdashelt, Bell and A. V. Sweigart~GSFC! improvedtheir population synthesis models of elliptical galaxies.new set of isochrones were calculated from Sweigart’s elutionary tracks and used to construct synthetic spectrastellar populations having ages of 4–20 Gyr and metalliciZ50.006, Z50.01716 and Z50.03. Synthetic spectra of fielstars of known effective temperature, surface gravity ametallicity were used to derive the color transformationeeded to put the colors measured from the synthetic spe

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onto the observers’ photometric systems. The newly traformed colors of the population synthesis models indicathat single, coeval, single-metallicity populations do notproduce the colors of early-type galaxies. For this reaswork is proceeding to examine whether the addition of bhorizontal branch stars and/or decreased mass loss alonasymptotic giant branch could resolve these color discrepcies. In addition, the treatment of TiO absorption in the sythetic spectra of M giants is being refined to more realiscally match the observed relationship between effecttemperature and spectral type.

5.4 Interstellar Medium and Star Formation

E. Ostriker, in collaboration with J. Stone and C. Gamm~CfA!, is continuing to model the evolution of dynamics anstructure in star-forming molecular clouds using numerimagnetohydrodynamics in 2.5 and 3 dimensions. Thsimulations survey observed molecular cloud conditionsvarying the amplitude of turbulence, the degree of segravity, and the magnitude of the mean magnetic pressrelative to thermal pressure. Results obtained indicatethe overall dynamical evolution of clouds is most sensitivetheir mean magnetization, secondarily sensitive to thmass, and least sensitive to their initial turbulence. Unmnetized model clouds, even with substantial initial turblence, collapse after about 5 million years at typical molelar cloud densities. Clouds with moderate magnetizat(;15m G! can survive for more than fifteen million years along as their total mass does not exceed the ‘‘magnetic ccal mass.’’ These results, together with results regardingdistribution of densities in model clouds of varying magnefield strengths, support the view that the magnetic procesare crucial to maintaining the ‘‘inefficient’’ star-formatioprocess which prevails in the present-day Galaxy.

Ostriker has investigated the acceleration and collimatof winds from accretion disks in Keplerian rotation, by thaction of large scale magnetic fields. She has obtained aparameter semi-analytic family of wind solutions in whicthe asymptotic constant-density and constant-velocity sfaces are concentric cylinders. The wind collimationachieved self-consistently by magnetic hoop stresses. Twind solutions show that it is difficult to achieve high-speoutflow and strong forward velocity direction in a selcollimated flow. This suggests that either~a! the collimationof observed ionized protostellar jets is primarily in densinot in both density and velocity, or~b! the observed protostellar jets are surrounded by a slow-moving wind which eforces the jet collimation. Ostriker has also joined facuVogel and Stone and student Nagar in modelling the outflfrom HH111. They show that the molecular kinematicsthis source are consistent with acceleration by a wide-anprotostellar wind.

With P. Teuben and graduate student J. Marshall, Ostrhas been investigating the dynamics of present-day sforming clusters with properties similar to the extensiveobserved Orion Nebula cluster. Their results, based on din-body simulations, have shown that the observed massregation and central density concentration could not hbeen achieved solely from two-body relaxation in a syst

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240 ANNUAL REPORT

of the presently-observed ONC mass within the knownmillion-year age of the cluster. This argues either for~a! anIMF which is position dependent within the cluster, or~b!significant changes in the cluster properties such as massdensity over its lifetime; further investigations will focus othe latter possibility.

M. Choi has used the BIMA millimeter array and thCSO submillimeter telescope to study the Herbig-Haroject HH 1-2. The study focused on the driving source foroutflow, and revealed a high-density, flattened core. In otwork, he has completed a survey of HCO1 emission fromten Class 0 protostellar objects using the BIMA array. Tprovided unique information on the protostellar collapmechanism. He is also observing several high-velocity oflows from young stellar objects using the CSO.

P. Safier has critically examined the physical basiscurrent magnetic-accretion models for classical T-Tastars. He argues that there is little observational evidencethe strong, dipolar stellar magnetic fields required by thmodels; also the steady-state assumed by these modelssupported by the models.

P. Safier has completed his study of the BD141 40 star-forming region with the BIMA millimeter-array. The imagereveal the structure of the associated molecular cloud,confirm the existence of a outflow source associated withcomplex. P. Safier and J. Lauroesch~NWU! are using theBIMA array at cm wavelengths in an effort to detect hydrgen recombination lines from quasars.

N. Nagar, S. Vogel, J. Stone, and E. Ostriker obserCO emission from the HH 111 optical jet with the BIMAarray. Both the CO kinematics and morphology indicatestriking hollow tubular structure surrounding the optical jThe velocity component parallel to the jet increases wdistance from the central star more rapidly than the comnent perpendicular to the jet. The data place constraintsjet-driven models for molecular outflows. The kinematiand structure are consistent with recent wide-angle, radwind models.

With P. Hofner ~U zu Koln!, L. Mundy and S. Vogel~UMD!, Xie imaged the CS emission from the UC HG35.2-1.74 with the BIMA array and the FCRAO 14m telscope, and found that the dynamics of the region is donated by a massive bipolar outflow oriented East-West wthe molecular gas significantly displaced from the UC HHowever, none of the existing theories for UC HIIs can sisfactorily explain the location and morphology of thsource. They propose that the presence of turbulent prescan explain the small size and longevity of UC HIIs.

Xie has investigated the scaling laws and Alfvenic manetic fluctuations in molecular clouds. Under the assumpthat the observed turbulent motions in molecular cloudsAfvenic waves or turbulence, he emphasizes that Dopbroadening of line profiles directly measure the velocity aplitude of the waves. Assuming equipartition, he propothat the observed scaling laws imply a roughly scaindependent fluctuating magnetic field, which might be uderstood as a result of strong wave-wave interactionssubsequent energy cascade.

T. Xie, L. Mundy, S. Vogel, and P. Hofner have bee

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working on confinement of Ultracompact~UC! H II regions.It has been proposed recently that the small size and llifetime of ultracompact~UC! H II regions could be due topressure confinement if the thermal pressure of the ambgas is higher than previous estimates. They point outconfinement by thermal pressure alone implies emissmeasures in excess of observed values. They show thatbulent pressure, inferred from observed nonthermal veloties, is sufficient to confine UC H II regions and explain thelongevity. They predict an anticorrelation between the sizeUC H II regions and the velocity dispersion of the ambieneutral gas and show that it is consistent with existing obsvations.

T. Xie and collaborators~Xie et al.1996! proposed a newmodel for the confinement of ultra-compact HII regio~UCHII’s!. They propose that turbulence associated withnon-thermal molecular line widths in the cores of clouds aas a pressure which balances the thermal overpressure iionized gas and keeps the HII regions from freely expandinto the molecular material. The model predicts a correlatbetween molecular line width and UCHII pressure whichconsistent with the limited data currently available. Addtional observations can provide further tests of the mode

S. Watt has begun studying the dust and gas distribuin and around ultra-compact HII regions with L. Mundy. Thobjectives of this work are to characterize the early stagemassive star formation and to understand the relationsbetween UCHII’s, hot cores, and star formation. The coplexity of this relationship is illustrated by the resultspaper by G. Blake~Caltech! L. Mundy and collaborators inwhich it is argued that the Orion hot core is externaheated. Much of the spectacular molecular emission whcharacterizes the Orion hot core, and identifies hot coregeneral, is argued to be arising from a layer of material beablated from the molecular core by nearby star formation

J. Stone’s research efforts are primarily concerned whydrodynamic and magnetohydrodynamic~MHD! studies ofthe ISM and star formation. Stone and graduate studenMiller are studying the interaction of a magnetized accretdisk with the magnetosphere of the central object. In the fiphase of the study, two-dimensional numerical MHD simlations have been performed to study accretion onto aligrotators with dipole fields. Using a variety of initial fielstrengths, geometries, and resistivities in the disk, rapidcretion which crushes the magnetosphere to the stellarface generally occurs. The study is being continued usthree-dimensional MHD simulations of accretion diswhich span many vertical scale heights to study the effecthe Balbus-Hawley instability in generating strong magnefield in the corona above the disk through dynamo actiand to study the interaction of this field with possible stelmagnetospheric fields.

Stone and physics graduate student T. Fleming are bening a study of the effect of microphysical resistivity on thsaturation of the Balbus-Hawley instability in weakly manetized accretion disks.

Stone has studied the nonlinear evolution and saturaof the Wardle instability in C-type MHD shocks in weaklionized regions of the ISM. The instability saturates as t

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sheets of dense gas orientated in the direction of shpropagation which anchor the field. In collaboration withNeufeld~JHU!, Stone has computed the temperature and pdicted the emission properties of unstable C-type shockcompare to observed systems. Surprisingly, the instabhas little effect on the emission properties of such shocprimarily because most of the emission occurs fromwhich is upstream of the strongest effects of the instabili

Stone, D. Proga, and J. Drew~Imperial College London!have studied the multi-dimensional structure and timdependent evolution of radiation driven winds from luimnous accretion disks. The winds are produced by radiapressure on Doppler-shifted spectral lines, similar towinds from hot stars. By adopting numerical techniques,assumptions about the geometry or time-evolution ofwind are required. In the case that the luminosity of the ddominates that from the central star, the winds are intrincally unsteady because the streamlines near the disk aretical, and the vertical component of gravity increases lineawith height so that it eventually chokes the wind. On tother hand, when the luminosity of the central star dominathat of the disk, the streamlines are inclined from the vertiin a way so that gravity is decreasing, and the flow is fouto be steady. The two-dimensional geometry, terminal veity, and mass loss rate in the wind are found to be remaably similar to the values inferred from observations of Csystems, and outflows from massive young stellar objestrengthening the idea that radiation driven disk windscrucial to understanding these systems.

Stone, Ostriker, and C. Gammie~CfA! are continuing tostudy the evolution of MHD turbulence in the cold ISMStone is leading the effort to perform fully three-dimensionMHD simulations of turbulence in a self-gravitating gas. Nonly can the damping rate of the turbulence be measuthrough simulation, but the density structures which rescan be directly compared to the observed structure ofclouds.

Wolfire is continuing his collaborations with various ISobserving programs. Along with C. Ceccarelli~Observatoirede Grenoble! they have presented the far infrared line sptrum of the protostar IRAS16293-2422 taken with the LWon board ISO~Ceccarelliet al. 1997!. They found a rich 43mm - 197 mm molecular spectrum dominated by@O I# 63mm and CO, H2O, and OH rotational lines. The@O I# emis-sion may arise from the compressional heating in the collaing envelope.

The initial spectrum of Supernova 1993J, which explodin M81, was classified as a type II supernova becauscontained hydrogen lines. Subsequently the hydrogen ldissappeared and its spectrum became similar to a typsupernova. Shortly after its first peak in optical brightneradio emission was detected from SN 1993J. W. K. Rosedeveloped a model for its radio emission that involvesinteraction of a collisionless shock front with the stellar wiof the progenitor supergiant. This model depends on emates of the mass loss rate of the stellar wind, the inveCompton effect and the apparent absence of synchroself- absorption.

Sage, Welch, and Lanzetta have begun a search for

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lecular absorption along the line of sight to a blank-fieradio quasar~NRAO 150!; no search for molecular emissioor absorption associated with known Lyman-a clouds hasbeen successful, but they hope this completely new approwill lead to the identification of dense gas associated wstructures~possibly proto-galaxies! in the early Universe.

5.5 Planetary Science

Lisse, Livengood, and A’Hearn, with colleague Lyncanalyzed MIRAC2 thermal infrared images of the P/SL-9impact site on Jupiter, taken; 2 days after collision. Aspectrophotometric search was made for various chemspecies, and a strong signature due to silicate emissionapparently found. However, this feature is unlike the typione found in comets, and the stellar calibration was probleatic on the day of observation due to inclement skies. A ncalibration procedure using the disk of Jupiter itself aVoyager 2 IRIS data is currently under trial.

T. Kostiuk ~NASA/GSFC! with colleagues Lisse and Livengood studied the auroral chemical signature of the nand south poles of Jupiter in the thermal infrared. Enhanhydrocarbon emissions from both North and South polargions as well as their global distributions were imaged. Dare being analyzed in support of ongoing studies of glostratospheric constituent distribution and of auroral morphogy and intensity.

Lisse and Livengood with colleague Kostiuk and studithe rings of Saturn in the thermal infrared. The rings weclearly detectable from 8 - 13 mm. A comparison of thespectral content on the East vs. the West rings indicatedistinction in emission intensity and spectral distributiosuggesting cooling of low thermal inertia particles on tnight side portion of the ring orbits. Follow-up observatioare planned for late 1998 to pursue this in greater detail.

Bus along with collaborators A’Hearn, Bowell and Stecompleted their study of the activity of Chiron at aphelioThey showed from archival survey plates that Chiron wclearly active at discovery as previously pointed out by oers and that the activity peaked in the early 1970s, shoafter aphelion. The intrinsic amount of brightening then wa factor two more than the brightening that was widely stuied on the approach to perihelion and which peaked in 19

5.5.1 Asteroids

Activities in the department related to asteroids focusaround preparation for and analysis of data from the flybyasteroid 253 Mathilde, a main-belt C-type asteroid with17.4 day rotation period, one of the slowest rates knownan asteroid. The NEAR spacecraft is on its way to aster433 Eros for a year in orbit around the mission’s primatarget.

In preparation for the Mathilde flyby and analysis of colimages in terms of possible mineralogical components, MFadden and colleagues Vilas and Jensen, convolved the stra of potential minerals likely to be found on the surfaceMathilde to the pass-bands of the filters in the NEAR spacraft’s Multi-Spectral Imager~MSI!. The authors found tha

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242 ANNUAL REPORT

a spatial unit on the surface of Mathilde must contain a mmum of some 55 asteroids and meteorites for it to betected by the MSI.

On June 27, 1997, the NEAR spacecraft passed wi1200 km of 253 Mathilde collecting hundreds of imagesthis C-type asteroid. Preliminary analysis of the data areappear in Science. With images of this asteroid, the scieteam, with L. McFadden a member, derived its size, 59 xkm, and its mean volume, 73,000 km3. A density rangingfrom 1.1-1.5 g/cm3 was derived by combination of Mathilde’s mass derived from telemetry. The asteroid’s visual gmetric albedo is 0.0361/20.002. Among Mathilde’s uniquefeatures, are 5 craters larger than 20 km diameter~approach-ing the radius of the asteroid! on the 60% of the asteroidviewed during the brief flyby.

5.5.2 Comets

Haken and A’Hearn completed their study~with Feldmanand Budzien! of the variation from one perihelion passageanother in the release of water from comets based onobservations of OH. Using comets whose orbital periodsnearly commensurate with that of Earth to eliminate sentivity to parameters in the models that are sensitive to geetry, they showed that the 2P/Encke varies by less than~1-sigma! per perihelion passage pre-perihelion whereasTempel 1 varies by a factor of 1.5 to 2 near perihelion froone passage to the next, apparently randomly, withousecular trend. The implications for mantle developmentcomets are important in that models must now explain tvery different types of behavior.

A’Hearn, in collaboration with the Rosetta-Alice UVspectrometer team used the HST-FOS to study the outsing of 46P/Wirtanen as it approached perihelion. Remaably, while the water release, as measured by OH, increaby orders of magnitude on the approach to perihelion,dust production, as measured by Afr, did not increase at allThe dust/gas ratio at the beginning of the observationsvery high by cometary standards but by the end of theserving, a month before perihelion, the dust/gas ratio wquite low.

Meier ~now at the IFA, University of Hawaii!, Wellnitz,A’Hearn and Kim~now at the Kyunghee Observatory, Korea! continued their analysis of the KPNO 4-m Echelle sptra of Comet C/Hyakutake which they had taken 1996 Ma26.3-26.5, at the time of closest approach of the comeEarth. At a spectral resolution of better than 0.2 Angstromtwo line multiplets of ND were separated from an overlayiNH line forest and used to derive a 3 supper limit for theD/H ratio in NH of 0.006, only slightly greater than threported detections at radio wavelengths. Although photoetry with an Echelle spectrometer is quite problematic, it wpossible to derive absolute production rates for water~fromthe OH bands! and for ammonia: Q~NH3!/Q~H2O!50.004.Single-cycle fluorescence models adequately describeobserved CH bands if two different rotational temperatuare introduced for the two spin-states: T~F1!5100 K andT~F2!5130 K; these temperatures are believed to be renants of a previous dissociative excitation, there not beenough time to completely relax CH before the radical

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cays. The rotational temperature may thus provide cluethe identity of the CH parents. A tentative detection ofhas been made in the spectra. This molecule had been pously noted in HST spectra taken two weeks later; the sond detection suggests that S2 is a persistent outgasproduct of comets.

Woodney continued her work studying sulfur chemisin comets. The first definitive detection of OCS in a comComet Hyakutake, was published by Woodney, A’HeaMcMullin ~now at NRAO!, and Samarsinha~now atNOAO!. This was a first step towards explaining the missisource of sulfur in comets, as H2S and CS2, the previouslyknown cometary sulfur sources could not account for allatomic sulfur observed in the UV. A minor species whopresence is difficult to explain through current chemicmodels of the coma, S2 , was also detected in this comet bA’Hearn et al., for only the second time in a comet.

More sulfur bearing species were detected in C/HaBopp than have ever been seen in any other comet, incluH2CS which was detected for the first time by WoodneA’Hearn, McMullin and Samarasinha. Analysis of the abudances of these species in comparison to the total atosulfur observed should reveal whether or not we can naccount for all of the primary sulfur sources in comets.

Woodney and A’Hearn were also a part of the muluniversity observing team which used the BIMA arraymap the comet in HCN, CS, CO and continuum at 3-mPreliminary comparison of HCN and CN morphology bWoodney suggests that HCN may not be the only sourceCN. The BIMA array was used by the team to discover tfirst mm-wave cometary ion, HCO1, in Hale-Bopp and itsspatial distribution is offset and much broader than thatother species mapped in the millimeter, such as HCN; sgesting production of the species in the plasma tail ofcomet.

Sherwin and A’Hearn studied the optical jets of com1P/Halley, using narrow band images from the InternatioHalley Watch data archives. Removing the continuum ctribution from the C2 and CN images, they found that thratio of CN/C2 flux was different by about a factor of 2 fotwo of the jets. The third jet’s CN/C2 ratio was not signifi-cantly different from the other two. The different ratios counot be explained by projection effects, or the different sclengths of the two species. The different CN/C2 ratios sug-gest that the material from which the jets originated also ha compositional difference.

Comet C/Hale-Bopp. A’Hearn, Fernandez, LissWellnitz, and Woodney led a coordinated effort to monitcomet C/Hale-Bopp at visible, infrared, submillimeter, aradio wavelengths throughout its apparition in 1996 - 19Photometric and spectroscopic observations were madetelescopes at Lowell, NASA/IRTF, BIMA, CSO, anNRAO. Very large amounts of dust were found in the comthe dustiest on record; sulfur species CS, SO, SO2, H2S andorganic species CH3OH, HCN, and CN were detected at etremely large production rates, with abundances; 1% ofH2O; an 11.3 hour rotation period for the brightest dustwas found; an outflow rate of; 0.4 km/sec was measurefor the emitted dust at 1AU. Work is proceeding on meas

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ing the effective infrared size of the nucleus, an estimexpected to improve the current 20 - 40 km radius estimderived from HST observations of the comet~assuming a 4%geometric albedo! Fernandez, A. Kundu, Lisse, and A’Heaused the VLA to detect the thermal radio continuum emsion from a comet for the first time. Fernandez and Wellnobtained an occultation of the 9th magnitude star P200723 by comet Hale-Bopp during an observation capaign in the American West run by the Lowell Observatoin October 1996. The shape and depth of the occultaprofile constrain the size of the comet’s nucleus to; 27 kmin radius, and show that the coma was optically thick out10-20 km from the nucleus. The comet group~what was leftat home! supported the education outreach activities ofdepartmental observatory during the comet’s perigee, duwhich some 3000 members of the public attended.

Comet 2P/Encke. The well studied comet 2P/Enckean extremely close approach to the Earth in July 1997,event happening once every 17 years. Fernandez, A’Heand Lisse observed this southern comet from the CTIO/Etelescopes in the optical and thermal infrared, in ordercharacterize the nucleus and dust emitted by the cometmultaneous infrared photometric measurements weretained with the ISO spacecraft. Imaging returned an unsolved point like structure to the comet, suggesting littleno emitted dust and gas, consistent with Encke’s knownactivity. Current work is bearing on the reanalysis of tcomet’s size estimate, thought to be; 2 km in radius, butpotentially much smaller in an analysis of July 1997 HSimages by colleague Weaver and A’Hearn.

5.5.3 Dust and Dynamics

D.P. Hamilton and colleague A.V. Krivov~St. PetersburgUniversity! discovered a new integral of the motion whicgoverns the evolution of planar orbits about planets whthey are influenced by four perturbations: i! solar gravity, ii!planetary oblateness, iii! solar radiation pressure, and iv!electromagnetism. Hamilton and Krivov extended the ingral into three dimensions, and showed that it is relatedthe Tisserand’s Constant of the restricted three body plem. They applied the integral to distant satellites of asoids and were able to infer simple properties of these orbFor example, distant prograde extend further along the Sasteroid line, while distant retrograde orbits extend furtperpendicular to this line.

D.P. Hamilton and A.V. Krivov also continue their woron dust around Mars. When the two small martian moonlPhobos and Deimos, are struck by interplanetary micromoroids, most of the impact ejecta is lofted into orbit arouthe red planet. Debris particles larger than several tenmicrons remain in orbit for tens to thousands of years andspread into broad rings by the action of Mars’ gravity asolar radiation pressure. Hamilton and Krivov performedtailed calculations of the number density of ring particles acompared their results to all other previous calculations. Trings are predicted to be very faint and have yet to be unbiguously detected; this may change in the next few yearspacecraft carrying sophisticated dust detectors reach M

Hamilton continued his work on the origin of the strikin

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black/white hemispheric asymmetry of Saturn’s odd satelIapetus. Iapetus’ orbital and spin period are equal, so itways keeps one face toward Saturn. It has long beenpected that dark dusty debris, originating from Saturn’s oermost satellite Phoebe and brought inward by PoyntiRobertson drag, is responsible for Iapetus’ striking albeasymmetry. The Phoebe-dust model is very compellingcause it naturally explains why the leading face of Iapetthe side that is receiving dust from Phoebe, is dark. Tmodel has not gained universal acceptance, however, prrily due to the following dynamical problems: i! the distri-bution of dark material on Iapetus does not precisely mapredicted contours of constant dust flux from Phoebe,!there are dark-floored craters in Iapetus’ high-albedo hesphere, and iii! Iapetus’ North pole is brighter than parts othe trailing hemisphere. Hamilton showed that problemsgreatly reduced with the realization that Iapetus took nearbillion years to become tidally locked to Saturn. A publiction is in preparation.

With E. Grun and the Galileo dust team, D.P. Hamiltohas continued his study of jovian dust streams: submicrsized grains accelerated away from Jupiter by strong elecmagnetic forces. Their analyses of new Galileo data and nmodeling efforts further constrain possible sources amechanisms. In addition, new concentrations of dust wdiscovered along the orbits of several of the Galilean salites, notably Ganymede. The dust sensed near the Galisatellites seems to be consistent with fragments ejecteding hypervelocity interplanetary impacts of interplanetamaterial with the moons. Data analysis and numerical meling efforts are underway.

Lisse, A’Hearn, and Fernandez used the Lowell aNASA/IRTF observatories to observe comet 126P/IRASthe optical and near-IR simultaneously with our ISO Gprogramme. The results have been used to characterizedust emitted by the comet as similar to that emitted byHalley with a 4:1 silicate:carbanaceous composition. Claboration with the MPIK ISOPHOT team of Gru¨en and Pe-schke has led to an on-going study of the dust emitted bother comets observed by ISO in the thermal infrared: 2Kopff, C/Hale-Bopp, 29P/Schwassmann-Wachmann 1,Chiron.

Periodic comets 22P/Kopff, 81P/Wild 2, and 46Wirtanen were observed serendipitously in the optical ainfrared by Lisse, Fernandez, and A’Hearn with colleagHanner~JPL! while monitoring comet C/Hale-Bopp. The results of these observations will be added to a growing infred photometric database of comets that will be used to cpare the dust emitted by comets for variations by adynamical class, and point of origin of the comet. The 81Wild 2 and 46P/Wirtanen measurements will also be deered to ESA and JPL to derive rough values for the dproduction rate and history of these comets, to aid in plning the upcoming ROSETTA and STARDUST comet redezvous missions.

Analysis in progress on MIRAC2 infrared observationsthe dust emitted by comet C/Hyakutake has demonstratedpreponderance of small, slightly hot silicaceous particlesan approximately 9:1 silicate:carbanaceous composition

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244 ANNUAL REPORT

a dust/gas ratio of; 2, similar to that of comet 1P/Halley.Lisse, A’Hearn,et al. published their comparison of th

dust emitted by the 4 comets~C/Okazaki-Levy-Rudenko, CAustin. P/Schwassmann-Wachmann-3, C/Levy! detected bythe COBE satellite. C/OLR and C/Austin were found to hamost of their dust mass in large, relatively cool, dark pticles, while comet Levy was more similar to comet Hallwith many small, hot particles. Comet P/SW-3 was probadetected due to an excess emission of dust related tobreakup next apparition. No comet trails were found inCOBE all sky survey, although particles which would bcome part of a trail were emitted from Comet Austin.

A’Hearn continued his collaboration with Lamy, Toth anWeaver to photometrically isolate cometary nuclei from tcoma using HST. Assuming canonical albedos, they ccluded that comets P/Wirtanen and P/Honda-Mrkos- Pdusakova were both substantially less than 1 km in radiand that they had relatively large active fractions of thsurfaces~10% or greater!. C/Hale-Bopp, on the other handhad a radius near 20 km based on a similar analysis.

Lisse, Fernandez, and A’Hearn used analysis of therinfrared images of comet C/Hyakutake during its perigeeMarch 1996 to search for emission from the nuclear surfaAlong with the 3 km~3a) upper limit for the nuclear sizefound from the VLA measurements of Fernandezet al., andthe unusually large optical scattering observed from ncoma sources, they were able to show that the most plauphysical model for the results has a comet with a 2.4effective radius nucleus, a tensile strength 13103 dynes/cm2,and a surrounding large cloud of slowly moving icy paticles.

Lisse used the ROSAT and EUVE spacecraft to obtphotometric imaging of comet P/Encke in the extreme ultviolet and soft x-ray. The comet was detected from 0.1 tokeV, by both spacecraft, and demonstrated an x-ray lcurve similar to the one found for C/Hyakutake in 1996, wboth a baseline slowly varying emission and an impulsoutburst; 4x the low level emission. Detections of comeC/Tabur and C/Hale-Bopp with colleagues at MPE, alowith archival findings by colleague Dennerl of 4 more RSAT comets and colleague Mumma of 2 EUVE archivcomets now brings the total of x-ray emitting comets;10, and demonstrates a rough linear correlation of x-luminosity with optical luminosity, and establishes that x-remission is a universal behavior of comets. Comparisonthe current observations to plausible physical models leathe following viable x-ray production mechanisms: chartransfer between heavy solar wind ions and cometary scies, bremsstrahlung scattering of energetic electronscometary neutrals, and solar magnetic field reconnecevents.

5.6 Space Plasma Physics

A. S. Sharma and K Papadopoulos have continued tinvestigation of the magnetosphere and space weather unonlinear dynamical technique.

The solar wind-magnetosphere system exhibits cohereon the global scale and such behavior can arise from nonearity in the dynamics. The observational time series d

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have been used extensively to analyze the magnetospdynamics by using the techniques of phase space reconstion. Analyses of the solar wind and auroral electrojet aDst indicates have shown low dimensionality of the dynaics and accurate prediction can be made with an input-oumodel. The predictability of the magnetosphere in spitethe apparent complexity arises from its being synchronizin the dynamical sense, to the solar wind. The strong etrodynamic coupling between the different regions of tmagnetosphere yields its coherent, and thus low dimensiobehavior. The data from multiple satellites and ground stions can be used to develop a spatio-temporal modelidentifies the coupling between the different regions. Thnonlinear dynamical models yield accurate and reliable focasting tools for space weather.

J. A. Valdivia ~former graduate student, now at NASGoddard!, A. S. Sharma and K. Papadopoulos worked onprediction of magnetic storms using nonlinear dynamimodels. The nonlinear dynamical technique of phase spreconstruction is applied to develop nonlinear modelsstorms and their predictability using the NGDC database1964–1990. Nonlinear predictive models based on thedata alone were developed. These models can predictstorm evolution consistently, and can identify intense storfrom moderate ones. The models based on IMF and Dst das well as those based on Dst alone, can be used for fcasting tools for space weather.

A. S. Sharma, J. A. Valdivia, S R. Rosa and H. S. Saw~both from INPE, San Paolo, Brazil! studied dissipativestructures and weak turbulence in the solar corona. Inwork for the first time the high resolution X-ray imagesthe solar corona, obtained by the Yohkoh Mission, as nlinear extended systems have analyzed. To qualifyspatio-temporal complexity in this extended system, thhave introduced an asymmetric spatial fragmentation pareter computed from a matrix representing an image. Choing different spatial scales on the same image, wave numare computed from the intensity contours and this yieldfragmentation spectra. This spectra is used to analyzeimages of the complex transient phenomena obtained bySoft X-ray Telescope board the Yohkoh satellite. The dnamics of the fine structures of the contours suggest thegin of the observed fragmentation to be the localized weturbulence processes occurring in regions with compplasma loop configuration.

A. S. Sharma and M. I. Sitnov~Moscow University, Rus-sia! studied the role of the temperature ratio in the linestability of quasi-neutral sheet tearing mode. Well-knowlinear stability criterion of the tearing mode in the magnetail current sheet with nonzero normal component ofequilibrium magnetic field consideres the electrons totrapped in the current sheet. This condition is significanmodified when electron-to-ion temperature ratio is small, dto the contribution of transient orbits to the perturbed eltron density integrated over the flux tube. In contrast toprevious criterion marginal stability may be reached unrealistic conditions in the near-Earth tail current sheet durlate substorm growth phase.

K. Papadopoulos, C. C. Goodrich and R. E. Lopez ha

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continued their work on the global modeling research. Othe last two or three years, they have made very significadvances in the understanding of the Earth’s magnetospand those of the outer planets. These advances have resfrom three fundamental improvements in their global MHcode, written by their collaborator, J. Lyon@Dartmouth Col-lege#, including extension of the range of the code to sevehundred Re downstream of the Earth, incorporation of tidependent conditions on ALL of the boundaries, and impmentation of a rotating dipole and semiconducting ionsphere. This work was made in collaboration with Dr. Lyand R. L. McNutt~Johns Hopkins/Applied Physics Lab!.

This group has also performed a series of global MHsimulations that may provide the key to understanding ilated magnetospheric substorms. This study has focusethe substorm event occurring at 0500 UT on March 9, 19which is particularly well documented by both ground basand spacecraft observations. It uses an impressive collecof ground radar, magnetometer, and other data which givthorough ionospheric view of the substorm. Data are aavailable from the ISTP spacecraft, which are well placedin situ observations. The simulation results give a globalscription of the substorm that is in good agreement with bthe ground based and in situ measurements. They find ooccurs about 0500 UT, following a growth phase markedthinning of the plasma sheet~PS! and heating of the PSplasma which starts around28 Re and then progresses taward. These results indicate onset is marked by an impulpenetration of electric field into the central plasma sheet,a diversion of the cross tail current, at about –10 Re. Thfeatures thereafter propagate down the tail to about –25where they engender strong reconnection, and the glmagnetic topology reconfiguration characteristic of the sstorm expansion phase. These results suggest a reamodel that combines the major aspects of the current leasubstorm models, the Near Earth Neutral Line and CurrDisruption models.

The SPP group has continued this activity in the ActAurora Research Program~HAARP!.

G. Milikh and A. Gurevich~Lebedev Physics InstituteMoscow, Russia! presented a new model of the artificial aglow due to the ionospheric modification. The modelbased on a recently developed nonlinear theory of stationstriations, which predicts a strong enhancement of the etron temperature inside the striations. According to ttheory the neutral species can be excited by the hot electinside the striations, which spread both upward and doward from the source region located between the radiowreflection and the upper hybrid resonance point. Thecussed model estimates the local electron temperature inthe striations, and the intensity of red- and green-line emsions due to the excitation of O~1D! and O~1S! levels by theelectron impact. These results were checked by comparwith the observations.

K. Papadopoulos, G. M. Milikh and J. Valdivia have cotinued their studies of high altitude lightning. Valdivia haworked on the problems related to this topic till spring 199when he graduated from the Maryland Ph. D. program. Kissues addressed were modeling of so called red sprites

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tical flashes located at 60–90 km above the thunderstowith emphasis on their fine structure, and study of the raway discharge in the presence of the geomagnetic fiBased on the fact that lightning discharges follow a tortuopath which have been identified as a fractal structures, tdeveloped a model of transient electric field from lightnias a fractal antenna. This model allows to obtain a spatemporal distribution of the optical flashes due to the ligning. They also presented a model of red sprite optical sptrum due to molecular excitation by ionospheric electroaccelerated by the electric field from lightning. The resuwere compared with observed red sprite spectra. Good agment with the observations was found for values of the eltric field close to the local breakdown threshold. Implicatioof the computations of the optical spectra to models ofsprites were also presented.

Another interesting effect related to the high altitulightning is the runaway electron breakdown which seemsbe a source of —ray flashes generated at the height in exof 30 km, which were recently observed by GRO obsertory. However, some attempts to explain this emissionglecting the effects caused by the geomagnetic field wincorrect as commented by Papadopouloset al. @GeophysRes. Lett., 23, 2283, 1996#. Thereupon K. Papadopoulos, GM. Milikh and J. Valdivia in cooperation with A. Gurevichproceeded with a new model, which generalizes the theorthe electron runaway discharge to the case of a laminar etric field at an arbitrary angle to the magnetic field, and drives the relevant threshold conditions. They showed thatconditions of runaway process depend on the angle betwthe of electric and magnetic fields, and the ratio of thmagnitudes. In fact, the geomagnetic field hinders the deopment of runaway breakdown in the atmosphere. Theyvealed that one can expect a significant difference inproperties of high altitude discharges occurs at equatoand polar latitudes.

G.M. Milikh, D.A. Usikov ~Phys. Department Universityof Maryland! and J. A. Valdivia presented a model of infrared emission from sprites. In this model the 4.26mm infraredemission due to read sprites is considered. The modelcusses generation of nitrogen vibrions due to the collisionthe nitrogen molecules with the electrons energized byelectric field from lightning, followed by transition of thenitrogen vibrions to the CO2~001! vibrational level with life-time much shorter than that of nitrogen. The infrared photoof wavelength54.26 mm radiated by the CO2~001! propa-gate through the optically thick atmosphere, therefore temission could be observed mainly from the space. Tmodel computes the infrared radiance of sprites, as welthe energy collected by the state-of-the-art space infraredtector, and estimates the signal-to-noise ratio at the dete

G. M. Milikh and J. Valdivia in cooperation with A.Gurevich delivered a model of X-ray emission and fast pconditioning during a thunderstorm. In this paper a quanttive self-consistent model of the processes determined bymultiple runaway breakdown caused by the thunderstoelectric field is presented. It describes in detail tbremsstrahlung emission and fast charge transfer,reaches a qualitative agreement with the earlier observat

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246 ANNUAL REPORT

obtained during a balloon flight through a thunderstorm. Tmodel demonstrates the exact realization of a fast chatransfer in a thunderstorm cloud. This process first predicby Gurevichet al. @Phys. Lett. A 165~1992! 364# is deter-mined by multiple runaway breakdown. It plays an importarole in the lightning triggering and has a strong influencethe development of a thunderstorm.

G. Milikh with A. Gurevich and N. Borisov~LebedevPhysics Institute, Moscow, Russia! continued their studies othe problems related to the creation of over-the-horizon raand new types of radio communication using an artificionized region in the atmosphere by means of powerfuradiation. Such a region acts like an artificial ionosphereflecting radio waves.

G. Milikh with G. S. Nusinovich~UMD! and B. Levush~Naval Research Lab.! discussed removal of halocarbonfrom air with high-power microwaves. They consider tdestruction of halocarbons caused by free electrons prodin a freely localized discharge initiated in the air by a fcused microwave beam. After fast cooling, the electronsattach to halocarbon molecules and destroy them. Operaregimes and their efficiencies were analyzed in the paand the corresponding parameters of microwave souwere specified. They also discussed the availability of micwave sources for such processes.

J. A. Valdivia and A. Fouladi~graduate student! intro-duced and implemented a new method of period controchaotic systems based on optimization. An error functiwhich is a measure of deviation from the desired time elution, is constructed. The error function is then minimizalong the trajectory in order to stabilize an unstable perioorbit. The process of optimization is not arbitrary but costrained to the dynamics. No specific knowledge of thesired state of the system is required. They also demonstrhow orbits of ‘‘high’’ period are similarly controlled.

K. Papadopoulos with A. Drobot and C. L. Chang~bothScience Application International Corp.! studied the physicsof current collection by the tethered satellite system whwas deployed from the space shuttle ‘‘Columbia’’ on Febary 24, 1996. Data collected revealed a host of new phyphenomena concerning the current collection by charbodies in space moving at orbital velocities. In fact, the crent collected was significantly larger than expected by spcharged limited flow in magnetized and even unmagnetiplasmas. The ongoing physics analysis of the observednomena was presented.

D. L. Book and J. A. Valdivia proposed that the differetial rotation of the Earth’s inner core, which was recendeduced, is due to a combination of the deacceleration ofEarth’s rotation and the viscous drag between the Earinner and outer cores. The proposed model allows to emate the dynamic viscosity in the inner core of the Earth104 poise. Besides providing a novel way of determiningviscosity of the core, this model suggests some new testsshows how astronomical effects can influence geologphenomena.

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5.7 Solar Radio Physics

M. R. Kundu and J. P. Raulin along with Nitta~LMSAL !,Shibata and Shimojo~NAOJ! have been searching for nonthermal radio signatures in the form of metric type III bursin conjunction with soft X-ray jets as observed by YohkoSXT experiment. In contrast to Anemone-type jets whenonthermal type III bursts were always observed, they founo evidence of type III bursts in association with two-sidloop type X-ray jets. This result is consistent with the simlation results of Yokoyama and Shibata~1995!, which showthat anemone type jets are produced by vertical/obliqplasma flow whereas the two-sided loop type jets are pduced by horizontal plasma flow.

Kundu, Shibasaki~Nobeyama! and Nitta~Lockheed! pro-vided the first evidence of the detection of microwave emsion at 17 GHz in association with coronal X-ray jets. Thpresented two typical cases one on the disk~1995 March 31!and the other at the limb~1992 August 25!. For the diskevent, they see 17 GHz emission from the upper part ofjet base~active region loop or loops!, but no emission fromthe collimated X-ray jet itself, implying that it must be optcally thin at 17 GHz. For the limb event, they see the basethe jet as well as the bottom part of the jet itself, implyinthat the optical depth is higher at the bottom part~obviouslybecause of higher electron density! than at the top. Theybelieve that the 17 GHz emission is thermal because igradual and unpolarized and that the heating processgives rise to the jet X-ray plasma also results in the 17 Gemission. The calculated 17 GHz flux densities seemagree with the observed values within a factor of 2. Tdisagreement is considered to be quite reasonalbe in viethe various uncertainties involved in computing the emissin both radio and X-rays.

Gopalswamy, Kundu, Lara along with Hanaoka, Enom~Nobeyama!, and Lemen ~Lockheed! presented X-ray~Yohkoh/SXT! and microwave~17 GHz Nobeyama! obser-vations of the 1993 July 10-11 CME. During this event,the substructures of a classical CME are revealed: Froloop in X-rays, prominence core in microwaves, dark cavbetween prominence and frontal loop in X-rays, and arcstructure beneath the prominence in X-rays.

Gopalswamy, Kundu, Hanaoka, Enome, Lemen and Aioka reported the observations of a coronal mass ejec~CME! using the Soft X-ray Telescope on board the YohkMission. The CME had the familiar three part structu~frontal loop, prominence core and a cavity!. The eruptingprominence was observed by the Nobeyama radioheliogrThey were able to determine the mass of the CME~2.631014 g! from X-ray observations which seems to be at tlower end of the range of CME masses reported before frwhite light observations. This is the first time the mass oCME has been determined from X-ray observations. Theight of onset of the CME was 0.3R( . The CME movedmuch faster than the erupting prominence while its acceletion was smaller than that of the erupting prominence.

Gopalswamy, Raulin, Kundu, Hildebrandt, Kru¨ger, andHofmann presented Very Large Array~VLA ! observationsof AR 7542 which demonstrate the existence of definite rand horse-shoe structures of a sunspot in intensity~I! and

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polarization~V! at 8.46 GHz~3.5 cm wavelength! and com-pare them with model calculations of gyroresonance ration. The VLA measurements have been made on threeferent days in July 1993 when AR 7542 was at thrdifferent longitudes which allows the study of the effectviewing angle on sunspot-associated microwave emissio

Model calculations of gyroresonance radiation have bcarried out using a modified dipole model correspondingthe observed photospheric magnetic field strength and aage temperature/electron density distributions consistentsoft X-ray and EUV observations~for the lower atmosphere!as well as theoretical assumptions~for the corona!. The cal-culated I and V maps were found to be generally consiswith the radio observations. They obtained informationthe magnetic scale length in vertical and horizontal dirtions above the sunspot and about the distribution of oplasma parameters~temperature, density! inside the radiosource region.

Gopalswamy, Kundu, Manoharan, Raoult, Nitta, aZarka observed an eruptive structure on 1994 July 31, a~900 km s21) in X-rays, followed by a slower plasmoid~180km s21). They were associated with a coronal mass ejectprominence eruption, and a host of metric radio bursts.X-ray structure seems to be a part of a white light coromass ejections~CME!, as inferred from the white light im-ages of July 30 and 31. A type II burst was observed atleading edge of the X- ray eruption, while a type IV burwas spatially associated with the detached plasmoid.type III radio bursts occurred on thin overdose structuassociated with the eruption. They detected the riseplasma levels because of mass addition to the type III bsources as a result of the eruption. This event further clarthe manifestation of a CME in X- rays. They identified thX-ray eruption as the driver of the coronal shock wave. Tprovides answer to the long-standing question regardingorigin of coronal and interplanetary shock waves. They afound evidence to support the idea that herringbone buare produced when the coronal shock wave crosses omagnetic field lines.

Gopalswamy, Kundu, Hanaoka, Enome and Lemen foremarkably different manifestations of a bright point flareX-ray and radio~microwave! wavelengths, unlike previouobservations. In X-rays, the BP flare was relatively simwhile in radio, the bright point flare had a large scale coponent and a transient moving component. The large sstructure may be the radio counterpart of large scale sttures sometimes seen during X-ray BP flares. The transcomponent was also compact and moved away from thecation of the X-ray BP flare with a speed of;60 km s21.The compact source also showed fast time structure wsuggests nonthermal emission mechanism for the transsource. The emission peaks in the two-wavelength domdid not coincide, which suggests cool plamsa flow alonglarge-scale radio structure. They were able to determinetemperature and emission measure of the BP flare plafrom the X-ray data, and thus they computed the expecradio flux from the X-ray-emitting plasma. They found ththe computed radio flux was much smaller than the toobserved radio flux.

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J. Zhang ~graduate student!, Gopalswamy, Kundu, ESchmahl, and Lemen presented the measurement of mnetic field gradient in magnetic loops in the solar coronbased on the multi-wavelength Very Large Array obsertions of two transient microwave brightenings~TMBs! in thesolar active region 7135. The events were observed at 2~spatial resolution;29) and 3.6 cm~spatial resolution;39)with a temporal resolution of 3.3 s in a time-sharing modSoft X-ray data~spatial resoltuion;2.59) were availablefrom the Soft X-ray Telescope on board the YOHKOH sellite. The three dimensional sructure of simple magneloops, where the transient brightenings occurred, were traout by these observations. The 2 cm and 3.6 cm sourcesvery compact, located near the footpoint of the magneloops seen in the X-ray images. For the two events repoin this paper, the projected angular separation betwencentroids of 2 and 3.6 cm sources come form the thergyro- resonance emission. The 2 cm emission is at theharmonic originating from the gyro-resonance layer whthe magnetic field is 1800 G. The 3.6 cm emission is at2nd harmonic, originating from the gyroresonance laywith a magnetic field of 1500 G. The estimaged magnefield gradient near the footpoint of the magnetic loop is ab0.09 G km21 and 0.12 G km21 for the two events. Thesevalues are smaller than those observed in the photospand chromosphere by a least a factor of 2.

Gopalswamy, Zhang, Kundu, Schmahl and Lemen carrout a detailed analysis of the Very Large Array data with ftime resolution~3.3 s! at two wavelengths~2 and 3.6 cm! andaddressed the question of the emission mechanism ofTMBs. They find that nonthermal processes indeed tplace during the TMBs. They present evidence for nonthmal emission in the form of temporal and spatial structurethe TMBs. The fast time structure cannot be explained bthermodynamic cooling time and therefore require a nonthmal process. Using the physical parameters obtained fX-ray and radio observations, they determined the magnfield parameters of the loop and estimate the energy releduring the TMBs. The impulsive components of TMBs imply an energy release rate of; 1.3 x 1022 erg s21 so that thethermal energy content of the TMBs could be less th;1024 erg.

Aschwanden and Benz~ETH Zurich! investigated elec-tron densities in solar flares. Soft X-ray emission measobservations from the Yohkoh satellite and plasma frequemeasurements with the Zurich radio spectrometers allowus to compare the electron density in soft-X-ray bright flaloops, in chromospheric evaporation upflows, and in acceration sites of electron beams. It was found that acceleratakes place in a region of much lower density than inflare loops, most likely located in the cusp region aboveflare loops, as envisioned in magnetic reconnection modThis work represents also the first density comparison wradio and soft X-ray methods.

Aschwanden, Bynum~Oregon College!, Kosugi~NOAO!,Hudson~ISAS! and Schwartz~GSFC! investigated electrontrapping times and trap densities in solar flare loops, usmeasurements from the CGRO and Yohkoh spacecrafsystematic timing study of energy-dependent time delays

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hard X-ray emission in the 20-200 keV energy range wperformed. It was found that the smoothly-varying hardray component revealed delays consistent with the collisiodeflection time. This result was interpreted that the hardray producing electrons are initially trapped in densitiesne'1011 cm23 and then pitch-angle scattered in the weadiffusion regime into the loss-cone, from where they precitate to the chromosphere and produce thick-target hard Xbremsstrahlung emission.

Aschwanden, Dennis~GSFC! and Benz~ETH Zurich!analyzed elementary time structures in solar flares, includsome 10,000 hard X-ray pulses, 4000 radio type III bur4000 pulses of decimetric pulsations, and 10,000 decimemillisecond spikes. The frequency distributions of the pefluxes of these elementary time structures were studiedeach flare separately, and were found to show power-lawwell as exponential distributions. The time profiles of eementary pulses could be fitted with a logistic curve, whallows to characterize the growth and saturation time ofunderlying nonlinear instability. With this logistic avalanchmodel, a generalized frequency distribution could be callated, which fits exponential distributions for incoherent pcesses, and power-law distributions for highly coherent pcesses.

E. Schmahl, C. Crannell~GSFC! and C. Wald~GoddardSummer Student, Columbia U.!, have investigated the extenof over resolution in solar flares using interferometers. Tmotivation for this investigation arose from results obtainwith the Owens Valley Radio Observatory Solar Arr~OVRO! and the Very Large Array. In many cases, pulished microwave events appear to be over resolved, sotimes with 50the total power being measured with the colated fluxes. Even the simplest event requires an additiolarger component, the flux from which is a factor of aboularger than the resolved component. In a sample of overflares from OVRO, we have found that flares are usuaover resolved even on the shortest baseline. This indicthat large-scale structures are common in solar flares, whas important implications for proposed hard X-ray image

Lee, White, and Kundu, along with A. N. McClymont~Uof Hawaii! and Z. Mikic ~SAIC! have completed the activregion study using the state-of-the-art coronal field extralation technique in the analysis of high resolution microwaimages of a solar active region. This field extrapolation futakes into account the nonuniform distribution of electcurrents observed in the photosphere and makes it possibinvestigate the link between regions of strong currentsthose of high temperature in the corona as implied by micwave images. They were able to reproduce the microwimages remarkably well with a model in which temperatuis structured along magnetic field lines, depends on therent on the field line, and increases with height in a mansimilar to that inferred from static heated loop models. Tresult implies a direct link between electric currents acoronal heating. They also used the magnetic field modeexplore a technique in which depolarization of microwavis used to infer the coronal electron density. It is shown tthis technique for obtaining coronal densities is particulasensitive to the quality of the coronal magnetic field mo

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which, in turn, depends significantly on the amount of eletric current used as a boundary condition for the field etrapolation. Such extreme sensitivity to the magnetic fimodel explains in part why the inference of a low density hbeen a common complaint of studies which have emplothe potential~current-free! field models to determine the density from the depolarization condition.

As an extension of the active region study, Lee and Whproposed a new diagnostic method for exploring 3-D mnetic field vectors in the corona. In the proposed methmagnetic field lines are traced in 3-D space to locate emsion layers for multiple frequencies used in the observatso that a magnetic field model can be tested by looking atcorrelation of the temperatures observed at different frequcies at the positions identified as the emission layers.proposed method relies only on the well-accepted theorycoronal loop structure and does not require ad-hoc assutions on plasma parameters.

Lee, White, and Kundu analyzed radio observations ofemerging flux region AR 7978 made using the VLA at twfrequencies, 5 and 8 GHz, with high time cadence. Thisthe first data set that shows coronal emissions associwith emerging flux detected from the very early stage ofemergence into the corona. From spectral analysis and cparison with EUV line images~FeIX/X and FeXII! obtainedfrom SoHO/EIT, they inferred that the emerging flux loopin fact, carrying heavier material inside compared with srounding corona, which contradicts the traditional view thsolar magnetic flux emergence is due to Parker’s instabfor magnetic buoyancy. It is also found that the radio flincrease is delayed by 1.5 hours with respect to emergencthe photospheric magnetic flux, and that the impulsive teperature increase is followed by that of density, consistwith the process known as ‘‘chromospheric evaporationEvidence for reconnection of an emerging loop to preexing loops is also found from radio images.

5.8 Instrumentation

Working with the HEIDI ~High Energy Imaging Device!team at Goddard Space Flight Center, Edward Schmahldeveloped a method for making quantitative charactertions of bi-grid Rotating Modulation Collimators~RMCs!that are used in a Fourier Transform X-ray imager. The teapplied the method to the RMCs on the HEIDI balloon paload in its preflight configuration. The results indicated t25’’ X-ray imaging optics on HEIDI are capable of achieing images near the theoretical limit and are not serioucompromised by imperfections in the grids.

Working with B. Dennis~GSFC!, E. Schmahl has produced simulations of the imaging capabilities of the pposed High Energy Solar Spectroscopic Imager~HESSI!.This Small Explorer will be a rotation modulation collimato~RMC! telescope with 9 collimators resolving angular scafrom 2.3 to 189 arcseconds, capable of mapping flares awhere on the Sun. The simulations have led to a numbeconclusions regarding the capabilities of HESSI for imaginspectroscopy. Among the conclusions are that 2-s mmade of flares with at least 1000 incident photons per comator per energy bin will show 50:1 dynamic range on s

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tial scales from 2.3 to 189 arcsec, and that gamma-ray~1-10MeV! flares can be imaged with spatial resolution of aban arc minute if they are spatially complex, and of orderarcsec if they are comprised of isolated point sources. Stroscopy of large~X-class! flares will provide spectral detailat the 1 keV level over at least the 5-100 keV range.

5.9 Other

Trimble has begun an investigation of the developmen20th century ideas concerning star formation. The most sting feature is a period from about 1935 to 1945 when mastream astronomical opinion denied that star formation wan ongoing process and confined it to the early period oevolving ~big bang! universe. Work continues on identifyinthe causes of this anomaly and the events that broughtan end.

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Aschwanden, M.J. and Treumann, R.A. 1997, ‘‘Coronal aInterplanetary Particle Beams,’’ G. Trottet~ed.!, LectureNotes in Physics, 483, 108-134~Berlin:Springer!.

Aschwanden, M.J., Dennis, B.R., and Benz, A.O. 19‘‘Logistic Avalanche Processes, Elementary Time Strutures, and Frequency Distributions in Solar Flares,’’ Apin press.

Aschwanden, M.J. 1997, ‘‘Nonthermal Flare Emissionchapter 8 inThe Many Faces of the Sun, A Summary ofthe Results from NASA’s Solar Maximum Mission, K.TStrong, J.L.R. Saba, B.M. Haisch~eds.!, Springer-Verlag,p. 265-292, in press.

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Aschwanden, M.J. 1996, ‘‘Do Electron Time-of-Flight Mesurements Support Magnetic Reconnection Models ?,’Proc. ofYohkoh Conference on Observations of Magne

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Blunt, V.M., Lin, H., Sorkhabi, O., A’Hearn, M.F., WeaverH.A., Arpigny, C., and Jackson, W.M., 1997, ‘‘UsinPhotochemistry to Explain the Formation and Observatof C2 in Comets,’’ Planet. & Space Sci., 45, 721-730.

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Bower, G.A., and 19 co-authors~inc. A.S. Wilson!, 1997,‘‘Kinematics of the Nuclear Ionized Gas in the RadGalaxy M84~NGC 4374!,’’ ApJL, in press.

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Bower, G.A., Wilson, A.S., Heckman, T.M., & RichstoneD.O., 1996, ‘‘Stellar Kinematics and Double-Peak

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Braatz, J.A., Wilson, A.S., & Henkel, C., 1996, ‘‘A Survefor H2O Megamasers in Active Galactic Nuclei – I. Observations,’’ ApJS, 106, 51.

Braatz, J.A., Wilson, A.S., & Henkel, C., 1997, ‘‘A Survefor H2O Megamasers in Active Galactic Nuclei – II. AComparison of Detected and Undetected GalaxieApJS, 110, 321.

Braatz, J.A., Wilson, A.S., & Henkel, C., 1996, ‘‘PowerfuH2O Maser Emission in LINERs,’’ inThe Physics of LIN-ERs in View of Recent Observations, M. Eracleous, A.Koratkar, C. Leitherer & L. Ho~eds.!, A.S.P. ConferenceSeries 103, 169.

Brandt, J.C., Lisse, C.M., and Yi, Y., 1997, ‘‘Small CurreSheets in the Solar Wind as the Possible CauseCometary X-ray Emission,’’ BAAS, 189, 25.05, Naturin preparation.

Brandt, J.C., A’Hearn, M.F., Randall, C.E., Schleicher, D.Shoemaker, E.M., and Stewart, A.I.F., 1996, ‘‘On the Eistence of Small Comets and Their Interactions with Plets,’’ Earth, Moon, & Planets, 72, 243-249.

Brandt, J.C., A’Hearn, M.F., Randall, C.E., Schleicher, D.Shoemaker, E.M., and Stewart, A.I.F., 1996, ‘‘SmComets~SCs!: An Unstudied Population in the Solar Sytem Inventory,’’ inCompleting the Inventory of the SolaSystem, T.W. Rettig and J.M. Hahn~eds.!, ASP Conf.Series Vol. 107, PASP, 289-297.‘

Bus, S.J., A’Hearn, M.F., Bowell, E., and Stern, S.A., 199‘‘2060 Chiron: Evidence of Activity Near Aphelion,’’Icarus, submitted.

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M. R. Kundu

1997 - Department of Astronomy - University of Maryland

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