Critical Nuclear Physics Needs for Astrophysical Nucleosynthesis Studies James W. Truran Physics Division Argonne National Laboratory and Department of Astronomy and Astrophysics University of Chicago March 23rd, 2010

Cosmic Nucleosynthesis Perspective The Universe emerged from the Big Bang with a composition consisting of hydrogen, helium, 2D, 3He, and 7Li. The first stars and galaxies were born with this primordial composition.

The heavy elements with which we are familiar - from carbon and oxygen, to iron, .. to uranium - are the products of nuclear processes associated with the evolution of stars and supernovae of Types Ia and II.

The classic papers by Burbidge, Burbidge, Fowler, and Hoyle (1957) and Cameron (1957) provided the perspective from which we now view this nucleosynthesis history.

The respective efforts by Cameron and Fowler to improve the nuclear physics input for nucleosynthesis studies largely reflect todays needs for both theory and experiment:

Fowler and his collaborators focused during the 1950s and 1960s on nuclear experiments involving light element reactions relevant to early stages of stellar energy generation - efforts which have established the empirical foundations of stellar evolution theory. Strongly motivated by Paul Merrills identification of the presence of technetium in red giant stars, Cameron focused rather on the mechanisms of heavy element synthesis and the need for theoretical estimates of reaction rates and other nuclear properties. CRL-41

Cosmic Abundances of the Elements Signatures of nuclear systematics:

-particle nuclei are dominant 12C --- 40Ca

dominance of unstable -nuclei decay products: 44Ti44Ca, 48Cr48Ti 52Fe52Cr, 56Ni56Fe 60Zn60Ni, 64Ge64Zn 68Se68Zn, 72Kr72Ge

nuclear statistical equilibrium centered on mass A=56 (56Ni in situ)

neutron shell structure reflected in the heavy element region at magic numbers N=50, 82, and 126 (s- and r-processes) Massive Stars & SNer-processs-process

Outline Helium Burning Reactions

CNO hydrogen burning in novae and X-ray bursts

Explosive Nucleosynthesis in supernovae: SNe Ia and SNe IISome interesting long lived radioactivities

Heavy element (A>60) nucleosynthesis mechanismss-processr-processp-processClues from abundance studies

12C synthesis realized in red giant (AGB) stars in incomplete helium burning regions (helium burning shells).Stellar helium burning proceeds mainly by the two reactions: 3 12C and 12C(,)16O.

+ 16O --> 12C + beamtargetbubblesSuperheated water will nucleate from a and 12C recoilsThe detector is insensitive to g-rays from the beam. The target density is 1000x higher than that of previous experiments. g-ray beam provided by the HIgS facility at Duke University.Water has been depleted in heavy isotopes of oxygen and deuterium. Backgrounds such as a-emitters can be rejected effectively.

First experiment has been scheduled for April 2010

OutlineHelium burning reactions

CNO Hydrogen Burning and breakout: Novae and X-ray Bursts

Explosive Nucleosynthesis in supernovae: SNe Ia and SNe IISome interesting long lived radioactivities Heavy element (A>60) nucleosynthesis mechanismss-processr-processp-processClues from abundance studies

Novae & X-Ray Bursts: Standard ModelsClassical Novae and X-Ray Bursters involve thermonuclear explosions in accreted hydrogen-rich envelopes of white dwarfs and neutron stars, respectively, in close binary systems.

Accretion of matter from their companion leads to growth of the envelope until a critical pressure is achieved at its base to trigger a thermonuclear runaway.

The violence of the ensuing outbursts depends strongly on the degree of degeneracy at the base of the envelope and the composition of the available nuclear fuel. The early stages of the outbursts of both novae and X-ray bursts witness rapid energy release on a dynamic time scale, moderated in a critical way by the operation of the hot (+-limited) CNO cycles.Nova V1974 Cygni 1992X-ray Burst GS 1826-24

Hot CNO Cycles and Breakout Reactions14O(,p)17F(p,g)18Ne(+)18F(p,)15OBreak-Out Reactions(T > 80 million K) CNO Cycles)+-Decay ConstrainedImplications for: novae and X-ray bursts 15O(a, g)19Ne(p,)20Na

*Nucleosynthesis Processesnp-processSupernovae?Supernovae

Courtesy: H. Schatz

Classical Nova Explosions under Extreme Conditions Townsley and Bildsten (2004) have shown that their exists a subclass of nova events occurring on massive white dwarfs accreting at extremely low rates and characterized by high densities and low central temperatures (T 5 million Kelvin).

Glasner and Truran (2009; 2010) have shown that such conditions lead to violent nova events with peak temperatures exceeding 400 million Kelvin, such that breakout from the conventional CNO cycles can occur. This may explain the anomalous abundance patterns seen in the ejecta of such systems.

Glasner and Truran have also demonstrated the sensitivities of breakout to critical reaction rates in the hot CNO sequences. We find a particularly strong dependence on the rate of the 15O(,)19Ne and a weaker sensitivity to the rate of the 14O(,)17F reaction. Also we note a constraint on flow past the iron peak at 64Ge.

(Glasner and Truran 2009)Breakout, the 15O(,)19Ne Rate, and Nova Abundances(The accreted matter was assumed to include no elements past oxygen.)(Glasner and Truran 2010)Note that heavier elements synthesized in nova events are observable in nebular remnants > a direct observational test.

Indirect (a,g) measurements

s(a,g)=for 15O(a,g)19Ne:ErJ Need: Gg(from Tg) Ga/GgTan et al. (2005), Kanungo et al. (2006) and Mythili et al. (2008) have measured G . We then also need a determination of Ga or Ga/Gg.

Davids et al. (2003), Rehm et al. (2003) and Tan et al. (2007) in observing the breakdown to 15O+ find Ga/Gg< 4x10-4. A more accurate determination is clearly necessary.

OutlineHelium burning reactions

CNO hydrogen burning in novae and X-ray bursts

Explosive Nucleosynthesis in SNe Ia and SNe II

Heavy element (A>60) nucleosynthesis mechanismss-processr-processp-processClues from abundance studies

Synthesis of Intermediate Mass and Iron-Peak Nuclei

Following helium burning, the elements from neon to zinc are formed in late stages of energy generation (carbon, neon, oxygen, and silicon burning) in massive stars (M 10 M) and in supernovae Type Ia and Type II.

The critical first step here involves the fusion reactions 12C(12C,p)23Na and 12C(12C,)20Ne.

Currently utilized reaction rates are obtained by extrapolation from higher energy ranges that do not involve resonances. Recent experimental investigations of the 12C+12C rate (e.g. Zickefoose et al. 2010) are focusing on extending the studied region down to the vicinity of the Gamow window. (See also Spillane et al. 2007).

Challenges remain to confirm the production levels of intermediate mass and iron nuclei formed under explosive burning conditions in Type Ia and Type II supernova events.

COSMIC ABUNDANCESThe abundance history of the intermediate mass nuclei Si-Ca relative to iron-peak nuclei (56Fe/56Ni) provides important constraints on SNe Ia and SNe II models.

Explosive Iron-Peak Nucleosynthesis Explosive nucleosynthesis is a complicated process. The nuclear products are sensitively dependent upon: thermonuclear reaction rates (level densities are critical), weak interaction rates (which serve to convert protons to neutrons - neutronization - and thereby reduce Ye and decrease the abundances of proton rich nuclei), and the (short) dynamic timescales appropriate to supernovae. Cameron (1963) recognized that the products of heavy element synthesis under explosive conditions were sensitive to the relative rates of strong and weak interactions and to the timescale on which synthesis occurred. This has ultimately led to the appreciation of the fact that the production of iron-peak nuclei in supernovae occurs under neutron-poor conditions (e.g. Ye ~ 0.5) such that the dominant products are proton-rich nuclei (e.g. 56Ni). Critical input nuclear physics includes experimental and theoretical rates and/or lifetimes for nuclei near the alpha-line, for both nuclear reactions and weak interactions.

Why 56Ni ? Pre-explosion compositions involve largely nuclei of Z N, viz. 12C, 16O, 28Si (for SNe II). Explosive burning at temperatures T > 4x109 K typically occurs on timescales seconds.

Supernova nucleosynthesis sees reactions occurring on a dynamical timescale. (Experimental rate determinations and Hauser-Feshbach calculations are critical input.)

Weak interactions proceed too slowly to convert any significant fraction of protons to neutrons. (Weak interaction rates are critical input.)

It follows that the main (in situ) iron-peak products of explosive nucleosynthesis in supernovae are proton-rich nuclei of ZN, viz. 44Ti, 48Cr, 52Fe, 56Ni, 60Zn, and 64Ge (Cameron 1963; Truran, Arnett, and Cameron 1967). 56Ni Production in Explosive Nucleosynthesis

Explosive Nucleosynthesis of Fe-Peak Nuclei 44Ti47V48Cr52Fe51Mn55Co43Sc40Ca60Zn65Ge64Ge68Se42Ca50Cr46Ti58Fe52Cr56Fe54Fe53Cr47Ti49Cr57Fe48Ti49Ti50Ti54Cr64Zn63Ga61Zn62Zn58Ni60Ni67Zn66Zn62Ni61Ni57Ni53Fe46Ca44Ca43Ca64Ni51V50V45Sc48Ca59Co55Mn70Ge72Ge65Cu63Cu59Cu68Zn72Kr45Ti67As66Ge69Ga71Ga70ZnZN56NiIron peak nucleosynthesis occurs at Ye0.5 in both SNe Ia and SNe II74Ge

Supernova Nucleosynthesis ContributionsType Ia Supernovae: Thermonuclear explosions of CO white dwarfs. Type II Supernovae: Core collapse driven events in massive stars. In both instances,the formation of iron peak elements in explosive nucleosynthesis occurs under neutron-poor conditions. This is reflected in the 56Ni56Co56Fe signatures in both Type Ia and Type II supernova light curves and in the isotopic compositions of iron-peak elements in Solar matter. (Iwamoto et al. 1999)(Thielemann et al. 1992)SNe 1987ASNe Ia

56Ni Production in SNe Ia / Nomoto56Ni (Ye 0)Neutron Rich MatterIntermediate Mass Elements: Si - Ca(Timmes, Brown, and Truran 2003)

Interesting Long Lived Radioactivities

Long lived radioactivities that are products of explosive nucleosynthesis (e.g. 7Be, 22Na, 26Al, 44Ti, 56Ni, 57Ni, 60Fe, .. 232Th,235U, 238U ..) can test and constrain models. Both 7Be (53.28 d) and 22Na (2.604 a) are anticipated products of classical nova explosions. Their abundance levels are sensitive functions of the convective/temperature history. The three long lived actinide isotopes 232Th,235U, 238U constitute important chronometers of the earliest stages of star formation activity in our Galaxy.

Supernovae represent the source of the interesting radioactivities 44Ti, 56Ni, and 57Ni in nucleosynthesis. Detections of gamma rays from 56Ni and 57Ni have informed us both of the mass of 56Fe in Type II supernova ejecta and of the 56Fe/57Fe ratio emerging from these events. We also understand the role of 56Ni in powering the light curves of Type Ia supernovae, utilized as distance indicators.

44Ti decay gamma rays have been detected from the Cas A Type II supernova remnant (see figure). The measured flux implies that if 44Ti and 56Ni were formed in a ratio consistent with the solar 44Ca/56Fe ratio, the Cas A supernova should have been brighter. Alternatively, the 44Ti may simply have been overabundant relative to mass 56 in that Type II event (the Ti/Ca ratio seen in halo stars is 3 times solar). Experimental determinations of the rates of production and destruction of 44Ti remain critical input.

Outline Helium burning reactions

CNO hydrogen burning in novae and X-ray bursts

Explosive Nucleosynthesis in supernovae: SNe Ia and SNe II Some interesting long lived radioactivities

Heavy Element (A>60) Nucleosynthesis Mechanismss-processr-processp-processClues from abundance studies

Neutron-Capture Processes of Heavy Element Synthesis

Our picture of the processes of heavy element synthesis has become considerably more complicated with time. Early work identified the s- and r-processes, with a relatively small contribution from the p-process.

We now understand that there are two distinguishable contributions (a weak and a main component) to both of the neutron capture processes: the s-process and the r-process.

The challenges to theorists include the need to identify the astrophysical sites in which this nucleosynthesis occurs and to explain the distinctive features of the diverse components. (Cameron 1963)

Neutron Capture Cross Sections: theory/experiment Truran and Cameron (1965)Macklin and Gibbons (1957)Rauscher, Thielemann and Kratz (1996)Thielemann, Arnould and Truran (1987)Bao and Kappeler (1986) Note systematic trends through positions of shell closures arising from level density variations.Neutron capture cross section uncertainties (Kppeler et al. 2007)

Identifying the r-Process Component in Solar System Matter Neutron Capture Cross Sections

CS 22892-052Search for a Second r-Process

The robust r-process abundance pattern in the regime from barium through the actinides does not extend below mass A 130, although the classic r-process model extends down to mass A 70. A second component is thus essential. Two possibilities are: the p-process (Frhlich 2007) an r-process associated with the mass shells ejected in Type II events weakr-process

*Nucleosynthesis Beyond Fe-group: p-processWith neutrinosWithout neutrinosProton-rich matter is ejected under the influence of neutrino interactions.Nuclei form at distances where a substantial antineutrino flux is present.True rp-process is limited by slow decays, e.g. (64Ge).Antineutrinos help bridging long waiting points via (n,p) reactions:(Frebel et al. 2005)

*Numerical Effects of Rate Variations(p,) ratesFrhlich, Tang, and Truran (2010)(n,p) rates

*5035363738394041424344454647484951525352Te106Te107Te108Te109Te110Te111Te11251Sb103Sb104Sb105Sb106Sb107Sb108Sb109Sb110Sb11150Sn100Sn101Sn102Sn103Sn104Sn105Sn106Sn107Sn108Sn109Sn11049In98In99In100In101In102In103In104In105In106In107In108In10948Cd97Cd98Cd99Cd100Cd101Cd102Cd103Cd104Cd105Cd106Cd107Cd10847Ag94Ag95Ag96Ag97Ag98Ag99Ag100Ag101Ag102Ag103Ag104Ag105Ag106Ag10746Pd91Pd92Pd93Pd94Pd95Pd96Pd97Pd98Pd99Pd100Pd101Pd102Pd103Pd104Pd105Pd10645Rh89Rh90Rh91Rh92Rh93Rh94Rh95Rh96Rh97Rh98Rh99Rh100Rh101Rh102Rh103Rh104Rh10544Ru87Ru88Ru89Ru90Ru91Ru92Ru93Ru94Ru95Ru96Ru97Ru98Ru99Ru100Ru101Ru102Ru103Ru10443Tc86Tc87Tc88Tc89Tc90Tc91Tc92Tc93Tc94Tc95Tc96Tc97Tc98Tc99Tc100Tc101Tc102Tc10342Mo84Mo85Mo86Mo87Mo88Mo89Mo90Mo91Mo92Mo93Mo94Mo95Mo96Mo97Mo98Mo99Mo10041Nb82Nb83Nb84Nb85Nb86Nb87Nb88Nb89Nb90Nb91Nb92Nb93Nb94Nb95Nb96Nb97Nb98Nb9940Zr80Zr81Zr82Zr83Zr84Zr85Zr86Zr87Zr88Zr89Zr90Zr91Zr92Zr93Zr94Zr95Zr96Zr9739Y78Y79Y80Y81Y82Y83Y84Y85Y86Y87Y88Y89Y90Y91Y92Y93Y94Y9538Sr73Sr74Sr75Sr76Sr77Sr78Sr79Sr80Sr81Sr82Sr83Sr84Sr85Sr86Sr87Sr88Sr89Sr90Sr91Sr9237Rb74Rb75Rb76Rb77Rb78Rb79Rb80Rb81Rb82Rb83Rb84Rb85Rb86Rb87Rb88Rb89Te105Ag93Tc85Mo83Nb81Zr78Zr79Y76Y77Rb72Rb73N = Z54555659605867Z = 50N = 50I109I110I111I112I113I108Xe110Xe111Xe112Xe113Xe114p process SHIPTRAPREFERENCE NUCLIDESJYFLTRAP(adapted from C. Weber)Penning Trap Mass Measurements (p=process regime)CANADIAN Penning TRAP at ANL

Main r-Process Nucleosynthesis Contributions Significant efforts have been made to understand the extremely robust r-process abundance pattern observed in the mass range from barium to uranium that characterizes both Solar matter and the oldest (and r-process rich) stars in our Galaxys halo.

This robustness strongly suggests perhaps even demands that significant fission recycling enables an approach to a stable pattern. This emphasizes the need for a more reliable treatment of fission including an understanding of the fragment distribution.

Critical nuclear input also includes masses (SN) for nuclei along the capture path, a more refined treatment of nuclear level densities, greater predictability of the behavior near shell closures, and a more extensive study of the beta-decay systematics of deformed nuclei (Mller, Nix, and Kratz 1997).

New CPT /Previous CPT measurementsCPT Fission Fragment MeasurementsOngoing program of measurements since March 2008, target 15 keV uncertainty38 species, 31 have never been measured by method other than -endpointTypically improved precision by factor of 5-10Adds to 30 measurements taken at CPT in past years with small gas catcher and source

R-Process ModelsSite-ing r-Process NucleosynthesisHelium Shellsof SNe II Neutron Star-Neutron StarCollisions Low Mass ( Prompt Expl.)SNe II- Driven WindsSNe IIIdentification of the r-process sites remains a major challenge.

Synthesizing the r-Process Mass Region A = 110-238 Farouqi et al. (2010) r-process calculations in the context of the high entropy wind model for two choices of mass model. The possibility of achievement of such high entropy conditions in Type II supernova environments remains a challenging issue (Panov and Janka 2009).

Calculated Fit to Solar r-Process Pattern

Kratz et al. (2004)

Synthesizing the r-Process Mass Region A = 110-238 The two most obvious constraints imposed on the r-process mechanism(s) by spectroscopic studies are: First that there exist at least two distinct astrophysical sites. Second that the process responsible for the synthesis of elements past Z = 56 is quite exttraordinarily robust. This points to fission recycling. Panov et al. (2004) provide beta-delayed and neutron-induced fission rates and argue that fission leads to the termination of the r-process. Petermann et al. (2009) have performed representative r-process calculations for different neutron-to-seed ratios that provide a measure of the importance of fission recycling to the robustness of this process.Panov et al. (2004)Petermann et al. (2009)

Fission Product Yields from Californium 252Cf Source at ANL78Ni (110 +100 -60 ms)130Cd (162 30 ms)(Hosmer et al. 2005)(Hannawald et al. 2001)

Concluding Remarks-I The nuclear physics input requirements for nucleosynthesis studies continue to reflect the importance of both experimental studies of identified critical reaction links and theoretical studies built upon statistical properties of nuclei.

In the light mass regime, the 12C(,)16O reaction is of particular importance. The triple-alpha reaction and the 12C + 12C reaction also warrant further experimental scrutiny.

Studies of breakout from the hot CNO cycles, relevant to both X-ray bursts and novae, will be greatly helped by an improved experimental determination of the rate of the critical 15O(,)19Ne reaction. Calculations of nucleosynthesis of Ne to Zn nuclei in massive stars and both SNe II and SNe Ia are sensitive both to nuclear reaction rates and to weak interaction rates. Both experimental and theoretical efforts are required. Observations of halo stars provide clues to and constraints upon the ratio of intermediate mass nuclei (Si-Ca) to iron peak nuclei while the range in peak brightness of SNe Ia also reflects variations in the 56Ni/(Si-Ca) abundance ratio.

Concluding Remarks-II In the regime past the iron peak, a number of processes can contribute. We now know that both the s-process and the r-process patterns demand contributions from at least two different sites.

A critical factor historically has been the separation of the s- and r-components using theory together with experimental neutron capture cross determinations. This may be more complicated in the mass regime A 60-90 where two s-process contributions may overlap. The dependence of the weak s-process on neutron capture cross sections has recently been examined by Pignatari et al. (2010).

The sites of the two (?) r-process contributions remain to be identified as do the range of physical conditions that they may reflect. Required experimental studies include both mass determinations on the proton-rich side relevant to the operation of the p-process in the mass range A 60-100 and those on the neutron-rich side relevant to determination of the beta-decay timescales at the waiting points along the r-process path in the vicinity of e.g. 130Cd, critical to the fission recycling that yields a robust r-process pattern in the extended mass range A 130-238 such as is observed in r-process rich and iron-poor halo stars. Further theoretical studies of beta-delayed and neutron-induced fission are also of interest.

Outline Helium burning reactions

CNO hydrogen burning in novae and X-ray bursts

Explosive Nucleosynthesis in supernovae: SNe Ia and SNe II Interesting long lived radioactivities

Heavy element (A>60) nucleosynthesis mechanismss-processr-processp-processClues from abundance studies

r-Process Neutron Capture Path

Heavy Element Abundances in Halo StarsMontes et al. (2007) here have identified two components of enrichment of r-process heavy elements in the early stages of Galactic evolution.

The r-Process Elements as Displayed in the Oldest Stars(Truran et al. 2002)These behaviors are compatible with nucleosynthesis occurring in SNe II.r-Process Abundances: CS 22892-06(Sneden et al. 2002)Here 5 r-process-rich stars robustly reproduce the Solar System r-process pattern in the high mass region but exhibit variations below ~ barium. Such stars reveal properties of single r-process events and Galactic r-process enrichment history (Truran et al. 2002).

Neutron Capture Cross Sections: theory/experiment Truran and Cameron (1965)Macklin and Gibbons (1957)Rauscher, Thielemann and Kratz (1996)Thielemann, Arnould and Truran (1987)Bao and Kappeler (1986) Note systematic trends through positions of shell closures arising from level density variations.

(Jordan et al. 2007)56Ni, SNe Ia, and the Accelerating Universe

Simulations of r processThanks to JINA for movie

Nickel Production in SNe Ia

Nuclear Physics Impact on XRB Light Curves Courtesy: H. Schatz

Solar System Iron Production The abundances of most of the iron-peak nuclei in Solar System matter were formed under proton rich (Ye 0.5) conditions in explosive nucleosynthesis.

This is reflected in isotopic production of even-Z elements. 48Ti and 49Ti formed in situ as 48Cr and 49Cr 50Cr as 50Cr: 52Cr and 53Cr formed as 52Fe and 53Fe 54Fe as 54Fe; 56Fe and 57Fe formed as 56Ni and 57Ni 58Ni as 58Ni; 60Ni,61Ni,62Ni formed as 60Zn,61Zn, and 62Zn 64Zn contributions from 64Ge ? 72Ge contributions from 72Kr ? Odd-Z: 51V (51Mn), 55Mn (55Co), 59Co (59Cu), 63Cu (63Ga)

The isotopic compositions of Cr, Fe, and Ni are derived from isotopes of different elements formed in explosive nucleosynthesis.

*Sensitivity of the p-processp-process nucleosynthesis depends on:Nuclear massesSpin and level information also importantRecent efforts with Canadian Penning Trap, JYFLTRAP, SHIPTRAPReaction rates(p,), (n,p), (n,), -decaysNeutrino induced reactionsAntineutrino charged current reactionsNeutral current neutrino reactionsThe figure reflects the nucleosynthesis patterns in stars of the lowest known metallicities with overabundances of elements through strontium. Frhlich, Tang, Truran (in prep) (Frebel et al. 2005)

Main r-Process Nucleosynthesis Contributions Significant efforts have been made to understand the extremely robust r-process abundance pattern observed in the mass range from barium to uranium that characterizes both Solar matter and the oldest (and r-process rich) stars in our Galaxys halo.

This robustness strongly suggests perhaps even demands that significant fission recycling enables an approach to a stable pattern. This emphasizes the need for a more reliable treatment of fission including an understanding of the fragment distribution.

Critical nuclear input also includes masses (SN) for nuclei along the capture path, a more refined treatment of nuclear level densities, greater predictability of the behavior near shell closures, and a more extensive study of the beta-decay systematics of deformed nuclei (Mller, Nix, and Kratz 1997).

Mass Measurement RequirementsFor r-process calculations, need mass uncertainty ~ 15 keV/c2 (1 : 107)Relevant nuclei are short-lived, do measurement in < 1 secondDo survey over many nuclides ( > 100)252Cf spontaneous fission yields78Ni (110 +100 -60 ms)

2-neutron separation energy: CPT results vs. FRDMApproximate r-process path

2-neutron separation energy: CPT results vs. AME03Approximate r-process path

*******(From NASA press release)

HUBBLE SEES CHANGES IN GAS SHELL AROUND NOVA CYGNI 1992 The European Space Agency's ESA Faint Object Camera utilizing thecorrective optics provided by NASA's COSTAR (Corrective OpticsSpace Telescope Axial Replacement), has given astronomers theirbest look yet at a rapidly ballooning bubble of gas blasted off astar. The shell surrounds Nova Cygni 1992, which erupted on February19, 1992.

... ********************(From NASA press release)

HUBBLE SEES CHANGES IN GAS SHELL AROUND NOVA CYGNI 1992 The European Space Agency's ESA Faint Object Camera utilizing thecorrective optics provided by NASA's COSTAR (Corrective OpticsSpace Telescope Axial Replacement), has given astronomers theirbest look yet at a rapidly ballooning bubble of gas blasted off astar. The shell surrounds Nova Cygni 1992, which erupted on February19, 1992.

... *****