ReferencesBarbuy, B.: 1988. Astron. Astrophys. 191,

121.Beynon, T.G. R.: 1978. Astron. Astrophys. 64,

145.Booth, A.J., Shallis, M.J., Wells, M.: 1983.

Monthly Not. Roy. Astron. Soc. 205, 191.Booth, A.J., Blackwell, D.E., Petford, A.D.,

Shallis, M.J.: 1984. Monthly Not. Roy. As­tran. Soc. 208, 147.

Franc;:ois, P.: 1988. Astron. Astrophys. 195,226.

Gratton, R. G.: 1988. In Stellar Evolution andDynamics of the Galactic Halo, M.Azzopardi and F. Matteucci eds., ESO,Garehing, 153.

Gustafsson, B.: 1988. In Stellar Evolution andDynamics of the Galactic Halo, M.Azzopardi and F. Matteucci eds., ESO,Garehing, 33.

Helfer, H. L., Wallerstein, G., Greenstein, J. L.:1959. Astrophys. J. 129, 700.

Lambert, D.: 1987. J. Astrophys. Astron. 2,103.

Magain, P.: 1987. Astron. Astrophys. 179,176.

Matteucci, F.: 1988. In Stellar Evolution andDynamics of the Galactic Halo, M.Azzopardi and F. Matteucci eds., ESO,Garehing, 609.

Sneden, C.: 1985. In Production and Distribu­tion of C, N and 0 Elements, J. Danziger,F. Matteucci and K. Kjär eds., ESO, Gar­ching,l.

Spite, M., Spite, F.: 1985. Ann. Rev. Astron.Astrophys. 23, 225.

Zinn, R., West, M.J.: 1984. Astrophys. J.Suppl. 55, 45.

this fact suggests a systematic differ­ence in the nucleosynthesis processesin globular clusters and field halo stars.

-0.2-0.6

+ Grotten et 01.-- field (Grotten)

-1.0

globular clusters were thus obtained.The results are displayed in Figure 3,which is analogous to Figure 1; eachpoint represents a globular cluster. Themean results for field halo stars are alsodisplayed here. Like fjeld halo stars, alsoglobular cluster stars exhibit an en­hanced odd-even effect with respect topopulation-I stars, approximately by thesame amount (- 0.3 dex) as field halostars. However, the results suggest thatManganese is more and more deficientas metallicity drops in globular clusters.If confirmed by more extensive results,

-1.4

[Fe/H]

-1.8

+ +++ ++

+ ++ +

+

-2.2

-------------------~--------+

Figure 3: [Fe/H} vs [Mn/Fe} for globular clusters. The solid line is representative for field halostars.

3.6-m telescope on La Silla. Thesespectra were used to obtain metal abun­dances which allowed the derivation ofa highly accurate metallicity scale forglobular clusters. The good correlationexisting between metal abundances ob­tained fram these spectra and photo­metrie indices like Q39 (Zinn and West,1984) is shown in Figure 2.

I performed an analysis similar to thatmade for the field halo stars on thesespectra. There are three good Manga­nese lines, near 6000 A, in these spec­tra. Mean abundances for seventeen

0.4

0.2

0.0,.......,~ -0.2"'-c~~

-0.4

-0.6

-0.8

-1.0-2.6

Chemistry at High Galactic Latitudes:CH, CH+ and CN Absorption LinesE. F. VAN OISHOECK, Calitomia Institute ot Technology Pasadena, USA, andC. P. OE VRIES, Laboratory tor Space Research, Leiden, the Netherlands

Introduction

The molecular clouds detected athigh galactic latitudes by Magnani, Blitzand Mundy (1985) through CO line emis­sion at 2.6 millimetre have attracted a lotof attention. They are particularly inter­esting because most of them appear tolie within the hot, local (d :s 150 pe)interstellar medium, where they may becondensing out of loops and filamentsof atomic hydrogen (Blitz 1988). Theclouds can also be characterized bytheir optical obscuration, and by theiremission at 100 Ilm as seen by IRAS (deVries and Le Poole 1985; Desert, Bazelland Boulanger 1988).

Although a considerable amount ofdata has been gathered over the pastfew years on the global praperties andmorphology of the clouds, still little isknown about the physical and chemicalstate of individual clouds. Because thehigh latitude clouds are embedded in adifferent environment, it is intriguing toask to what extent they differ from the"classical" diffuse clouds (such as the~ Oph or ~ Per clouds), or the "classical"dark clouds (such as the Taurusmolecular cloud TMC 1 or L 134 N). Forexample, the high latitude clouds havelow visual extinctions, Av = 1-2 mag,similar to those of the diffuse clouds that

have been studied extensively by opticalabsorption line observations; yet theirstrang CO millimetre emission is muchmore characteristic of that found in darkclouds. Are the abundances of othersimple moleeules also enhanced in highlatitude clouds compared with diffuseclouds? If so, what pracess is causingthis enhancement?

Observations and Results

In order to investigate this question,we praposed to search for molecularabsorption lines in Southern highlatitude clouds using the Coude Echelle

47

WAVELENGTH (A)Figure 1: Absorption spectra of HO 210121 and HO 147889 in the wavelength region of theinterstellar CN and CH absorption lines. The line of sight towards HO 210121 passes through ahigh latitude cloud; that towards HO 147889 through a translucent cloud. The broaddepression in the HO 210121 spectrum is the Hel 3888 Aline that arises in the atmosphere ofthe star itself. Oifferences in relative CN and CH abundances in the two clouds are apparentfrom these spectra. The individual CN and CH lines are indicated, and for CN, lines arising outofN = 0 and 1 are detected. In the HO 210121 cloud, the resulting CN excitation temperatureTex = 2,85 K is consistent with excitation in the 2.7 K cosmic background radiation field only.For the HO 147889 cloud, Tex> 3 K is found, which suggests that local excitation effects playarole as weil. In the case of CH, we looked primarily

for the A2 -x2n (0,0) band at 4300.3 Ä.Lines of interstellar CH with equivalentwidths in excess of 1 mÄ were detectedin the spectra of about 10 stars. Thestrongest CH line with W)" = 22 mÄ wasfound for the line of sight towards HD210121, a bright B5-6 V star rv = 7.5;E(B-V) = 0.32; I = 56°.9, b = -44°.5)which is fortuitously located behind thethickest part of a high latitude cloud.The CH measurements are of specialimportance, because the CH abun­dance has been found to be weil corre­lated with the H2 column density in inter­stellar clouds (Danks et al. 1984; Mattila1986). They thus allow an independent

place in July 1987 and 1988, and De­cember 1987. Because little informationis available on the reddening towardsthe stars, we first looked briefly at theinterstellar Na D lines at 5890 A to ob­tain an impression of the amount of in­terstellar matter in front of the stars.Stars which show strong, saturated in­terstellar Na lines in their spectra withequivalent widths W)" larger than100 mÄ clearly lie behind the cloud;stars with weak or no Na Lines lie infront of the cloud. If the distances to thestars are known from photometry, theNa line observations can also be used toplace limits on the distances to theclouds (Hobbs et al. 1986; Andreani etal. 1988). Lines of interstellar CH, CHiand/or CN were sought towards about25 stars which had strong interstellar Naabsorption. More details of the observa­tions and reduction can be found in deVries and van Dishoeck (1988).

During our first two observing runs,one of us (CPdV) was present at La Silla;the third run in July 1988 was our firstexperience with remote observing fromGarching. On the whole, we encoun­tered only minor problems due to thefact that we were more than 10,000 kmaway from the telescope, although theobservations were slightly less efficient,because it took a bit more time to pointthe telescope. We think remote obser­vations are a good alternative for goingto La Silla if the observation period isrelatively short (a few nights). For longerperiods however, remote observing wasexperienced to be far more exhaustingthan observing at La Silla because of theweird times you are busy, due to the 6hours time difference with La Silla. How­ever, we noticed one significant disad­vantage in Garching compared with LaSilla, namely the food service! Fortu­nately, aur integration times were usual­Iy long enough to allow us sufficient timeto go to Garching in the mornings toshop for our next meal ...

eH

3890

HD 147889

3885

vations towards high latitude stars canbe seen from the beautiful Na spectrapresented by Andreani et al. in the previ­ous issue of the Messenger.

The Southern high latitude c10udsstudied in this work were not taken fromthe Magnani et al. catalogue, but werechosen on the basis of IRAS 100 flm skyf1ux maps. Background stars wereselected by comparison with the cata­logue of stellar identifications (CSI).Eight different clouds with a total ofabout 40 background stars wereselected for observations, which took

38803875

R(O)

CN0.6

Spectrometer (CES) on the ESO CATtelescope. One observational problem isthat only few bright, early-type stars areavailable as background light sources athigh latitudes. However, the improvedsensitivity of the CCD detector, com­bined with the high spectral resolution(AIö-'f... = 60,000), makes the CES auniquely qualified instrument to obtainhigh quality spectra towards less brightrv = 8-9 mag), somewhat later-typestars (Iatest we observed were late A­type). The suitability of the CES foratomic interstellar absorption line obser-

>- 1.0

E-i~

rJ)

Z~

P(l)E-iZ CH R 2 (1)~

H 0.8 R(l)

~

>- 1.0E-i~

rJ)

z~ CH R 2(1) CH Q2(1)E-iZ P(l)~

~R(l)

~ 0.8

R(O)

HD 210121CN

3875 3880 3885 3890

WAVELENGTH (A)

48

TAßlE 1. Comparison of Ihe HO 210121 cloud wilh olher clouds

Species H0210121 ~ Oph HO 169454 HO 147889

H2 ........... (0.6-1.0) (21) 4.2(20) (1-2)(21) > 2(21)H ............ (1-5)(20) 5.2(20) ~ 9(19) -CH ........... 3.5(13) 2.5(13) 4.6(13) 8.0(13)

CW .......... 6.0(12) 2.9(13) - 3.0(13)CN ........... 1.4(13) 2.9(12) 5.5(13) 1.5(13)CO ........... (0.5-1.5) (16) 2.0(15) (1-9)(16) > 1 (16)

Av(mag) ....... 1.0 0.9 3.5 -4

Note: The table lists column densities in cm-2. The notation a(b) indicates a x 10b

.

determination of the H2 column densi­ties in high latitude clouds, a subject ofconsiderable controversy. Estimates ofH2 column densities from integrated COmillimetre line intensities usually adoptconversion factors that have been cali­brated from Giant Molecular Clouds inthe Galaxy, and it is not clear apriorithat these factors are also appropriateto high latitude clouds. For the cloud infront of HO 210121, four differentmethods for determining the H2 content(CH abundance, CO millimetre emis­sion, extinction, and IRAS 100 flm flux)have been compared by de Vries andvan Oishoeck (1988). The various es­timates suggest that most of the hydro­gen is in molecular form in this c1oud.

Quite surprisingly, the CW ion is notvery abundant in high latitude clouds.The CW A1 n-X1L:+ (0,0) line at 4232.5 Awas detected (W1• > 1 mAl towards only3 of the 10 stars observed. These obser­vations are interesting because CH+ isthought to be formed only in shock­heated gas. The fact that the CWabundance is comparatively low sug­gests that shocks do not playa domi­nant role in the chemistry in the cloud.This is in contrast with the conclusion ofMagnani, Blitz and Wouterloot (1988),who had conjectured that shocks mightbe important based on large OH andH2CO abundances found from radiomeasurements.

CN

Ouring our most recent observing runin July 1988, we also searched for linesof CN in the violet B2L:+ -X2L:+ systemaround 3875 A, where the response ofthe CCO detector is still good. Stronginterstellar lines were detected in thespectra of 4 of the 5 observed stars.Figure 1 shows the observed spectrumtowards HO 210121. As a bonus, thespectrum also includes lines of the near­by CH B2L:--x2 n (0,0) system around3890 A. Although these CH lines are in­trinsically weaker than the A-X 4300.3 Aline, they provide an independent checkon the inferred column density.

The strongest CN line is the R(O) line,which has an equivalent width of about25 mA towards HO 210121. Unfortu­nately, this line is strongly saturated, sothat the determination of column den­sities requires some assumption aboutthe velocity spread parameter b in thecloud. One method to obtain an esti­mate of b is to compare the strengths ofthe R(1) and P(1) lines, wh ich are alsoclearly visible in the spectrum. Theselines originate in the same lower level N= 1 of the molecule, and should there-

fore give the same column density. ForHO 210121, the R(1) and P(1) data are inharmony for b = 1 km S-1, a value that isconsistent with the width of the COmillimetre line towards the star (Knappet al. 1988). A more powerful method forconstraining both the CN column densi­ty and the value of b is to compare theCN violet system results with measure­ments of CN in the red A2 1l-X2L:+ (2,0)system around 7900 A (cf. van Oishoeckand Black 1988 a; see also theMessenger 38, 16). Searches for the CNred system towards HO 210121 re­vealed no interstellar lines with W1• >2 mA. This negative result immediatelyprovides an upper limit on the CN col­umn density in the lower N = 0 level of1013 cm-2

, and suggests that b cannotbe smaller than 1 km S-1. Thus also in­formation on the "turbulence" in highlatitude clouds can be obtained fromthese observations, wh ich may providefurther insight into their origin.

Once the column density in N = 0 isweil constrained, the remaining CN datacan be used to determine the excitationof the molecule. The observed relativepopulations in the N = 0 and N = 1 levelscan be characterized by an excitationtemperature Tex (1-0) = (2.85 ± 0.25) K.The upper limits on lines out of N = 2result in Tex (2-1) $ 4.7 K. Thus the CNexcitation in the HO 210121 cloud iscontrolled by the 2.7 K cosmicbackground radiation, and local effectsplaya negligible role. This conclusion, inturn, provides limits on the electron den­sity in the cloud.

Comparison With Other Clouds

How do the CH, CH+ and CN resultsfor the high latitude clouds comparewith those for c1assical diffuse and darkclouds? Table 1 lists the observed col­umn densities for the HO 210121 cloudtogether with those for the ~ Oph diffusecloud. Table 1 also includes the resultsfor two thicker, so-called translucentclouds with visual extinctions of about3 mag (van Oishoeck and Black 1988 a).The observed spectrum of the transIu­cent cloud in front of HO 147889 around

3880 A is reproduced in Figure 1 asweil. From this figure, it is clear that therelative abundances of CN and CH differgreatly in the two clouds: the CN linesare about equally strong in the twospectra, but CH is clearly relativelymuch strongertowards HO 147889 thantowards HO 210121. From Table 1, itappears that the CH column density isonly slightly larger in the HO 210121cloud compared with the ~ Oph cloud,whereas the CH+ column density is evensmaller. On the other hand, both CN andCO are more abundant in the highlatitude cloud, although their columndensities are not yet as large as thosefound in translucent clouds.

Oetailed steady-state models are cur­rently being developed to interpret thesefindings (van Oishoeck and Black1988 a, b). Initial results suggest that thelarge CO and CN abundances most like­Iy result from the fact that the highlatitude clouds are exposed to less in­tense ultraviolet radiation than clouds inthe galactic plane. The relatively low CHabundance is most easily explained ifcarbon is more depleted onto grains inhigh latitude clouds compared withdiffuse clouds. Finally, as mentioned be­fore, the small amount of CH+ suggeststhat shock processes playa less impor­tant role in the chemistry of high latitudeclouds compared with diffuse or transIu­cent clouds.

In our future absorption line observa­tions, we intend to investigate whethergradients occur in relative abundancesacross a cloud in cases where we haveidentified several background stars.Also, we intend to search for millimetreemission of several other molecules totest the chemistry further and to con­strain the physical conditions. Insummary, a considerable amount of in­formation on the origin and the physicaland chemical state of these interestinghigh latitude clouds can be obtainedfrom combined optical and millimetreobservations.

References

Andreani, P., Ferlet, R., lallement, R., andVidal-Madjar, A. 1988, The Messenger, 52,47.

49

Blitz, L. 1988, in "Structure 01 Galaxies",Proc. 10th European Regional Meeting 01the lAU, ed. J. Palous (Czech Academy 01Sciences, Prague), p. 204.

Danks, A. C., Federman, S. R. and Lambert,D. L. 1984, Astron. Astrophys. 130, 62.

Desert, F.X., Bazell, 0., and Boulanger, F.1988, preprint.

de Vries, C. P. and Le Poole, R. 1985, Astron.Astrophys., 145, L7.

de Vries, C.P. and van Dishoeck, E.F. 1988,Astron. Astrophys., in press; and in prepa­ration.

Hobbs, L. M., Blitz, L., and Magnani, L. 1986,Astrophys. J (Letters), 306, L109.

Knapp, G. R., Drdla, K., and van Dishoeck,E. F. 1988, in preparation.

Magnani, L., Blitz, L., and Mundy, L. 1985,Astrophys. J, 295, 402.

Magnani, L., Blitz, L., and Wouterloot, J.G.A.

1988, Astrophys. J, 326, 909.Mattila, K. 1986, Astron. Astrophys., 160,

157.van Dishoeck, E.F. and Black, J.H. 1988a,

Astrophys. J., submitted.van Dishoeck, E. F. and Black, J. H. 1988 b, in

Rate Goefficients in Astrochemistry, eds.T.J. Miliar and D.A. Williams (Kluwer, Dor­drecht), in press.

Evolutionary Features in Distant Clusters of GalaxiesA. BUZZONI, E. MOLINARI, Osservatorio Astronomico di Brera, MilanoI. MANOUSOYANNAKI, Istituto di Fisica Cosmica deI CNR, Milano, andG. CHINCARINI, Osservatorio Astronomico di Brera and Universita di Milano, Italy

present some of the preliminary resultswe have obtained; the observationshave been carried out at La Silla and theanalysis in Milano where, with the helpof the ESO staff, we have installed theneeded software for the data reduction.

Figure 1: The apparent size of the galaxiesversus redshift. A galaxy size of25 Kpc and aHubble constant Ho = 50 are assumed. Eachcurve is labelIed with the value of the decele­ration parameter qo' As a reference also thelinear dimensions on our GGO frames arereported (the scale is 0.675 arcseclpixel).

and Gunn, 1976, Wade et al. 1979) andthe EFOSC focal reducer.

The images were reduced using theMIOAS package (see Users Guide, Im­age Processing Group ESO V 4.3) in­stalled in the reduction facilities of theObservatory of Brera while the detectionof the objects in each frame and theirphotometric measurements have beencarried out using the INVENTORY soft­ware (West and Kruszewski, 1981) im­plemented in MIOAS. We are confidentthat our completeness is down to themagnitude r = 23.5 as it is apparent froma direct inspection of the features de­tected automatically in the CCO images.The uncertainty in the photometry of thefaintest objects detected should be lessthan ± 0.3 magnitudes (for details seeMolinari, 1988).

The two clusters are shown in Fig­ures 2 and 3. These images have beenobtained by adding the CCO frames atour disposal in the r band and are equi­valent to an integration time of 50minutes for 2158 + 0351 and 85 minutesfor 0020 + 0407. In Figure 4 we havemarked all the objects photometered in2158 + 0351 just as an example of thecompleteness attained. No seriousattempt has been pursued to discrimi­nate between stars and galaxies usingthe analysis of the photometric gradientof the single features. This is because atsuch redshifts the galaxies are expectedto be essentially seeing limited imagesand cover only a few pixels on the CCOframe. As shown also in Figure 5, thebrightest members of 2158 + 0351 are8 x 8 pixels wide and obviously the situ­ation gets worse for the more distantcluster. It is because of this and thedifficulty to always have subarcsec see­ing that we decided to use a statisticalapproach to the photometric informa­tion available (magnitudes and colours).

05

RED HIF'To

o

2. Observations

We observed two high redshift clus­ters taken from the survey published byGunn, Hoessel and Oke (1986): 2158 +0351 at a nominal redshift of z = 0.445and 0020 + 0407 at z = 0.689. Theredshifts were taken at their face valueand should eventually be confirmed.

A set of CCO frames were collectedduring two observational runs at theCassegrain focus of the ESO 3.6-m tele­scope in November 1986 andNovember 1987. We used the three­colour Gunn photometric system (Thuan

1. Introduction

The study of distant clusters of galax­ies is fascinating and fundamental for avariety of astrophysical problems rang­ing from the understanding of the evolu­tion of the galaxies as a function of thecosmic time to the comprehension ofthe large scale structure of the Universe.Relevant contributions on this topicoccurred in the recent years, followingthe pioneering work of Gunn and col­laborators (Gunn, Hoessel and Oke,1986) in the detection of clusters at highredshift, of Butcher and Oemler (1984) inemphasizing possible evolutionaryeffects present in distant galaxies and ofTinsley (1977) with her theoretical work.

No matter what our goals are, in ob­servational cosmology we are alwaysfaced with the problem of using galaxiesas tracers of the geometry of space,measuring their position in the sky andestimating their luminosity and distance.This is a relatively simple task if we referto the nearby region of space, say, atcosmological distances smaller than z =

0.2. Here galaxies are still relativelybright and extended objects and theycan be recognized and easily separatedfrom stars. However, as soon as we lookdeeper in space, attempting to studygalaxies located farther away, we startdealing with almost unresolved faint ob­jects and it becomes difficult both tomeasure redshifts and estimate uncon­taminated counts as a function of themagnitude (Fig. 1).

A way around the problem may be touse a combined approach by means ofa multicolour photometry (Loh and Spil­lar, 1986, Rakos, Fiala and Schombert,1988) properly matched with a set ofevolutionary photometric models ofgalaxies. With this in mind, we startedsome time aga with a long term projecton distant clusters. In this paper we

50