to a well-defined observational limit and In a letter of February 20,1074 to the [10] A more msive descrlptlon Is in the specifying main characteristics such as Dimtot General of ESO, Holmberg mlnutes af the Instr. Comm, of March Hubble type and apparent magnitude? wrote fhat, since November 1973, the 1964, In FHA. Many considerations pointed to an- work had hngoing full force hy Andris [ll] Mltlons du Cenh Natlonal de Re- mheScieniiffque, Paris 1990, p. 404. swering w,including the important hub-, and a flrst batch of 20 plates [I21 Ses ESO Annual Reports 1964-1866 side effect of ensuring uniformity in the were under survey. A comp~nslve and mhutes Cw Meetlngs 1965 and identification numbers to be used in the description of the project was published 1966, In FHA. future. in 1974 by Holmberg, Laurn, Schus- 113'j FHA-Cou minutes Dec. 1968, p. 4. Since the task would be far beyond ter and west 1251. 1141 l-teckmann Stems, p. 21 6 and 321 -322. what might be done by the ESO staff [15] In a letter of January 10, 1990, Prof. U. Haug of Hamburg Observatory points M,collaboration with an astronomical Acknowledgement Institute, preferably in one of the ESO out to me, that In the a of the Hm- cwntrie, would be the solution and this Iam indebted to Ors. R B. Mulbr and burg Schmidt, whereas Strewindti was responalMe for the mechanical Wlgn of R. M. West helpful on a led the ESO Directomte to approach in for comments the mounting, the combination optlcs- the spring of 1973 the Director of Upp- draft of this article. telaseope tube was primarily handled by sala Obssrvatury, Eric Holmbrg. Upp- ZeiwJena, induding a solution for the sala Obmatwy was one of the few In References and Notes ailgnrnent te~copetubdgulding-We- the ESO antries with an established Abbmviatims used; =w=. tradition In extragalactic work, including EC = €SO Committee, the committw that [lq Ref. PI, p 79. work of statistical nature. A major pro- pseceded the ESO Cwncll. [I7] "Asfronomy WNh Schmidt Tekwps", ject published in 1973 was P. Nilson's EHA - ESO Historical Archives. See the & Ed. M. Capaceloti, Reidel, 1083, p. 13. Uppsala General Catalogue of Galaxies, wripdon in the -No, 54 of Oe- [is] Mitteiiungm Ver. Mbank-Fahiken No. 15, March 1958, p, 1, In EHA-HI. containing data for nearly 13,000 galax- cember 19&8. WA - Am belonging to the Office of the [19j AecoKlingtoaletterbyO.SfianetoJ.H. ies north of declination -2"30' and Head of Adrninistratlon of ESO. Oort of August 22, 1960; In EHA- based on the Palomar Sky Suwey 1231. EHPA - ESO Hlsori~alFltotogrephs Afchlver. IA.1.18. In reply to a formal letter of May 16, Heckrnann Sterns - 0. Heckmann, %me, (aq See note 19). 1973 of the ESO Director General, Kosmos, Wehmodelle, Verlag Plper and pl] Documentation pertaining to the de- Holmberg expressed his interest in the Go., MIlnchen - Zurich, 1976. velopment of the ESO-SRC oollabora- proposition and sketched first outlines [I] I am Indebted tu Prof. U. Hrwg of Ham- tlon is contained in FHA-2.8.3, 'Cooper- for the collaboration In a letter of May burg Observatory for prwfdlrlg me with ation with s%", Including eoph oi meor- respondence between West, Blaauw 27. Further correspondence and meet- the references PI and Dl below. 121 Abhandlungen Hamburger Stmwarte and Reddish and the legal advlsors of ings between €SO and Uppsala staff Id Band X, Heft 2, p. 50, 1979. ESO and SRC from Aprll20, 1QM and to a formal agreement between the two !3ee 0. Hackmann, in Nahrre, Vol. 76, draft texts for the Agreement fm institutes of February 8, 1874 g4]. p. 805,1955 and In Mitteilungsn Asbvn. November 1972 till the find vadon of In the course of the negotiations, for GewlI~chaft1955, p. 57,1956. Jenuary 1974. ESO the Head of the Sky Atlas Labora- 141 See Fehrenbach's report In the minutes [aSee, for Instance, the internal Memo ref. tory, Richard West, became more and oi the 9th mating of the Instr. Cwnm., 6W74/18B/RW/FP of October 10, 1974 more involved, and $con took this pr* Oct 18,1963, p. 10 in FHA. Remnce is from west to various EN meen: m st ject, too, under his wings. The agree- also mde to the minutes of the EC of d plates which have been dlstribuw In ment spwlfied, among other items, that Nov. 1961, Oct. I-, In FHA, and to the EM-Ill. p3] UppsaIaAstron. Qh.Ann., Vd. 6,1873. the Uppsala search was to be made by ESO Annual Rep. 1984. [q Mlnuteg of the 6th meeting of the Instr. 1241 I am mueh Indebted to Prof. E. Holm- an astronomer at Uppsala Obsmvabw Comm., p. 3, in FHA. berg and Dr. A. tau- for pmvldlng on copies of the original plates of the Minutes Instr. Camm. June 25, tQM, me with ~~piwof the dyCOW Quick Blue Suwey especially made for p. 7, in FHA spondence In the film of Uppsala Ob- this purpose; an Annex, apart from giv- m Minutas of the 13th meeting of the Instr. swatow: letters of May 27 and Sept. ing technical details, stated that besides Cmm., p. 4: Minutm 2nd Cou Meeting, 26, 1973. The ESO FHA-2.8.6. contaln, galaxies satisfying certain observational May 1984, both In FHA for the pariad reported here, eopb of criteria, also a selection of stellar clus- t8] See atso, In PIA-IA2.10, relevant eor- correspondence and drafts M well as the Sept. ters and planetary nebulae were to be respondance between Oort and Heck- final conkact, beginning 26, 1373. See also the €SO Annual Reports. included. The criteria to be adoptd for rnann In June and July 1984 and Mh 1966. [2q E.B. Holmberg, A. tauberts, H.-E. the selection of the galaxles were tha [9] Pmeedlngs d the Conference an "The S~husterand R. M. West, The €SONpp- same as thme used by Nilson so that Role of Schrnldt Telescopes In As- sala Survey of the ESU (6) Atk of the homogeneous coverage of the northern tronomy", Ed. U. Haug, publlWd jolntly Southern Sky. I., In Astm. Astrophys. and swthem pat% of the sky would be by ESO, SRC and Hamburg Obsmato- Suppl18, p. 463-489,1974. assured. v,1972, p. 137-139.

Open Clusters Under the Microscope 6. NNODSTRdM and J. A NDERSEN, Copenhagen University Observatory, Denmark

Stellar Evolution Models theory has provided a framework within Stellar evolution models are used. to whbh, in broad terms, we can fit the calculate the ages of observed stars and Our use of, and faith in. stdlar evolu- apparently bewildering variety of single their lifetlmea in various evolutionary tion models underlie mueh of contern- and double stars into a logical order phases. They also describe the transfor- wary astrophysics. Stellar evolution described by a physlcal theory. mation of llghter to heavier chemical elements withln the stam, and the amounts of prmessed material returned to the Interstellar environment at various stages during their evolution. Thus, stel- lar evolution models are an essential basis for models of galactic chemlcal evolution, a subject of much current in- terest. How well do these models corre- spond to the d stars7 Open clusters are an excellent piam to make the corn- parison, but care is required in interpret- ing what one sees. We have attempted to look a bit deeper Into this question than b often done.

Weak Points Among the weak points of current stellar evolutlon models is their treat- ment of the energy transport in the stel- lar interior. At the microscople level, the effectof the absorption, mission, and scatter- ing processes encountered by a photon on its way towards the surface of the star is described by opacity tables. 15 Here, the 1970 Cox-Stewart (C-S) 0.3 0.5 0.7 0.9 1 .1 1.3 1.5 opacities have now been superseded by B-V the more recent Los Atamos Opacity Figure 1 : CM diagmn of lC 4851, kmAntAony-Twmg et a/. (1988). lsmhmes rrOm M& Library (LAOL). The LAOL opacities are, (1990) are shown, for the obmwd reddening and metal abundmca. Dotted: #a omding, on average, significantly larger than 22 10' y~ Solid: WN overshooting, 4,O I@ yr. those by C-S, leading to cooler and less lumtnous stellar models (see Guenther et al., 1989). At the macroscopic level, the treat- (whlch might also have evolved) match ter determining the evolutlon of a star) ment of convective energy transport in those of the observed star? on a model isochrone, the locus far stellar interiors has long hnrag- Wdl, for which real stars do we know models of different mass, but the same nbd as one of the weakegt points in the mass, radius, effective, temperature, age. stellar evolution theory. Standard mud- chemical composition, and age, accu- Wise masses and radii of eclipsing els use the classical mlxing-length ap- rately and without reference to stellar binary stam for such comparisons have proximation, but it has long been argued models7 The answer is: Only for one long been a pet subject of ours. Here, that the conwlve motions will "over- star, the Sun. So, the mlnimum require- we shall just quote a couple of relevant shoot" lnto the classically stable, radb- ment for a trustworthy evolution code is recent results: Standard models (Van- tive regions. Overshooting from the con- to produce a satisfactory solar model. In denbrg, 1985) gave a superb M to the vective cores of massive stars Increases fact, no set of standard solar models evolved system Al Phe at 1.2 sotar the amount of hydrogen fuel avaitable in has been found to account for all of the masses when LAOL (but not C-S) the main-sequence stage. Hence, main- observations mentioned above and for opacities were used (Andersen et al., sequence models with convective over- the obsenred solar neutrino flux, oscilla- 1988). However, at just slightly larger shooting are brighter than standard tion spectrum, and surface lithium masses (1.5 -2.5 sotar masses), binary modds, and the stars leave the main abundance as well (Sackmann et al., stars near the top of the main sequence sequence wRh higher ages and larger 1990). But how about stam of other can only be fitted reasonably by over- helium cores, modifying thelr later masses and ages? shooting models (Andersen et al., 1990). evolution. Later precise studies of additional The Binary Test binaries in this mass range confirm this Models vs. Real Stars conclusion. But what do the clusters tell Apart from the Sun, two alternative us? As no satisfactory physical over- kinds of test object exist: eclipsing shooting thwry exists (Renzini, 1Q87), binaries and star dusters. The former Its existence and eventual importance have the great advantage that their The Cluster Test must b~ ascertained by comparison masses, radii, and effective tempera- As test objects for stellar evolutlon with real stars (Chtosl, 1990; Maeder, tures can be determined with great models, star clustem have the advan- 1890). So let us assume that we have a accuracy in a fundamental manner. Met- tage of populating the entire isochrone, real star of known mass, composition, al abundances can be determined by defining Its shape much more precisely and age, and use our favourite evolution standard spectroscopic methods. The than possible with the mere two points code to construct a computer model of ages am not known, but must be the supplled by a binaty system. For this this star. How well do the observable same for both components. Thus, one reason, cluster colwr-magnitude (CM) properties, radius, effedve temperature binary system provldm us with two diagrams have been compared exten- or luminosity, and surface composition points of known mass (the key parame- sively with theoretical model isochrones closely in two open clusters of inter- For distant clusters such as IC 4651, mdlate age, IC 4651 and NGC 3680. accurate radial velocities (1 km s-I or Our results show that these 'nitty-gritty better) are the most reliable indicator of details" are far from inconsequential membership: If repeated observations when one wishes to actually test stellar show a constant vetocity equal to the models: The purpose of a critical test Is cluster mean, there is a vwy strong not to play with enough free parameters probability that the star is bath single to tit the data with one's favourhe mod- and member of the cluster. el, but b see whether at least some of Figure 2 shows what the "micro- the wmpeftng models are In fact ex- scope" reveals after two seasons ofab- cluded by the data. sewations: The large majority of the stars do indeed appear to be single cluster stars. Granted, a ooupb may turn out to be long-period bidesIn the IC 4651 is a fairly rich open cluster cluster, which we happened to observe with turnoff stars of mid-F typ. The just when they were close to the mean Frgw 2: Close-up of the drcted wion of ob- most recent photometric studies of It velocity. Sure, one or two may be field TII, whew CQMVEL radial-veIoclty with the same velocity as ssrvatlons haw bean mads. Dots: LlkW sin- are by Anthony-Twamg et al. (1988) and stars nearly gle members. Crosses: EHablished nm Nissen (1988), who found its age to be IC 4651. But cewinly not all the stars in members. Circle: Un&n (bmd-lined about 2.5 1On yr, based on the Vanden- Figure 2 are blnaries and nen-membm, star). Berg (1985) standard models. Mazzei as would be required for the standard and Pi tto (1988) dedved an age of isocfirone to be the appropriate choice. 1.3 10F yr with modsls incorporating IC4651 does appmr to show unambig- strong overshooting, which they found uous evidence for the presenm of con- (e-g. Vandenkg, 1985), and quite pre- to be superior to standard models. vective core ovmhooting. ciw age detenninationa have been re Maeder (1Q90),on the other hand, de- ported (kg. Mazzel and Pigatto, 1988). rived ages of 4 10' yr and 2.2 10' yr, We concentrate in the following on the respectively, from models with and age range covered by the open clusters. without overshooting - from the same NGC 3680 Is a cluster very similar to As tools for testing stellar models, CM CM diagram! lC 4651 In age and metal abundance diagrams of star clusters also have a Which of all these wildly conflicting (both marginally higher than IC 4651). number of drawbcks: First, of course, estimates can one believe? And, to ask We show its CM diagram in Figure 3, the true masses of the stars are In prin- the underlying key question, does the assembled from the photometry of ciple unknown. Second, defining the CM diagram of IC 465 t provide unam- Anthony-Twarog et al. (1989), Nissen correct effective temperature and biguous evldence for significant over- (1 988). and Eggen (1969). The CM dia- lumlnosiiy scales in the cluster requires shooting In the cluster stars, or does it gram is less neat than that in Figure l, that the reddenlng and distance of the not? due to the larger Influence of field stars. duster have been accurately deter- The CM diagram of IC 4651 Is shown A striking feature is the apparent mined. Next, the metal abundance must in Rgure 1. The isochrones are those by dichotomy of the main sequence into be accurately measured by photometric Maeder (1 WO),fitted with the reddenlng two parallel sequences, the so-called and spectroscopic obsmations of clus- and (solar) metallicity determined direct- "blmodal turnoff discussed extensively ter stars. Then, some cluster stars wlll ly by Nissen (1988). Clearly, the main by bath Nissen (1988) and Anthony- be unresolved binaries and appearing differences between the two curves are Twarog et al. (1 989). No credible expla- brighter and generally redder than the In the upper part of the turnoff: The nation for a ma! feature of this type mom Iuminous component by itself (If overshooting isochrone extends signifi- could be found. both are main-sequence stars). Finally, cantly higher above the turnoff than the Again, we have fac~ed our most cluster CM dlagrams also contain standard lochrone, and also curves CORAVEL "microscope" on the encir- a signiicant number of non-member or gently towards the red before reaching cled stars in Figure 3. Although our field stars. the "red hook". 1989- 1990 observations of NGC 3680 All of these efFects combine to pro- The issue Is decided by the true na- are much more complete than those of duce more ambiguity in fitting thmretl- ture of the encircled group'of stam Are lC4851, what we see is not qulte as cal imhrones to observations than of- they all binary and field stars which clear-cut as that In Figure 2. This Is be- ten meets the eye in published dia- should be disregarded? If so, the stan- cause the fraction of stars with broad gm.If reddening, metal abundance, dard models without oversht>otlhg and lines and/or variable or slightly discrep- and distance have not been determined the associated (low) age are to be pre- ant vdocitles is much larger. More data separately, but included in the tit, any ferred. Or are they mostly slngle cluster are needed to clearly separate the ad- systematic emin the model colours members? In that case, the standard ow categories of stars. we expect to or luminosities wlll be hidden, but isochrone is clearly not an acceptable obtain these during the I Wt obgerving reappear as systematic errors in the de- match to the observations, and the high season. However, several robust con- rived ages. Emwill also murif red- age estrmate must prevail. cluslons are already emrwglng: dening or metal abundance detmtna- Rrst, about 2/3 (I) of the stars in Fig- based ure3 appear not to be members of tlons were parlly on non-member IC 4651 Mlcmmpe stars. under the NGC 3680 at J. Also, binaries we Further uncertainty in the interpreta- Let us look for the answer by putting rabr frequent among both cluster and tbn of details in the isochwnes is intro- the turnoff region of IC 4651 under the fleld stars. Then, when binaries are Iden- duced If, in addltion, one can pick and mkroscope. Our "microscop" in this tifid and field stars removed, the choose which stars to exclude from the case is the radial-vebciity scanner "bimodl turnoff diwlminto a single fit as presumed blnaries and rmn-mem- CORAVEL (Mayor, 1985), mounted on sequence of stars. Interestingly, this se- krs. We have examined this point the Danlsh 1.5-rn telescope at h Silla. quence shows the shallow slope on the uppef main sequence and the large ver- 9 tical extent and redward curvature of the turnoff which distinguish the overshoot- ing isochrone in Figure 1: Overshooting seems definitely present in NGC 3680 , also.

Some Lessons One lesson is immediately obvious: If significant conclusions on stellar mod- > 1 2 els or cluster age depend on the correct Interpretation of minor featuree of a cluster CM diagram, careful star-by-star identification of non-rnernbms and , blnaries is essential. The microscope may reveal that Initlal, and perhaps biased, Impressions are in fact wrong. Then, together with the binary evi- dence, the cluster data seem to show that convective overshoofing does exist 1 5 as an observable phenomenon in stars 0.2 0.3 0.4 0.5 0.6 0.7 0.8 of these masses. Future stellar evolution b-y models jnto have to take this F~um3: CM diagram of NGC 3680 Irom Anthwry-Twarog et a/. (1889), N(ssen fl988), md account. With their precise photometry Eggen (1968, transformed to by). hte the appamnt bimodal& of the meh seqmce; and membership clusters such as COWEL data for the clrcled stars show thls to be an ar#8ct of Mn#y and m-membsr IC 4651 and NGC 3680 will be very vat- mtam/nath, uable In helping to calibrate the mode! mameters used in the convection we- &ription. We hope to extend this &e of work to clwets of other ages, and sub- maybe because the C-S opacities lead Anthony-Twarog, BJ., MuWaesjee, K., Cat& to their modds being too hot. Over- well, C.N., and Twarog, BA.: 1988, AJ., ject new generations of stellar models to 95, 1453. tests based on both binary and cluster correcting for reddening by 0.1 5 mag will, of coursB, lead to significantly lower Anthony-Twarog, B.J., Twarog, BA, and data. Shodhan, S.: 1989, A.J., 98, 1634. A third significant consequence of age estimates. Chlosi. C.: f990. w.Ask. Soc. m. 102. accepting the valid& of overshooting 41 2: models is that higher ages (-4 10' yr) Acknowledgements Eggen, OJ.: I-,&. J. 168,439. are estimated for the stars in IC 4651 Guenther, EB., A. Jaffe, Demarque, P.: 1989, and NGC 3680 than with standard mod- We thank Drs. B. Anthony-Twarog, Ap J. SQB, 1022. A: els, by almost a factor of two (cf. Fig. 1). M. Mayor, B. Twarog, and 0. Vanden- Maeder, t 990, in Astrophysid Ages and Berg for helpful conversations and cor- Dating Mettlods, E. VangionCFlam, This result appears to be typical for M. CaM, J. Audolae, J. Tran Thanh-Van, evolved main-sequence stars (cf. respondence, and ESO, the Danish Board for Astronomical Research, the Ed. FmntiBm, Pa&, p. 71. Table I in Andersen et al., IWO),and Mayor, M.: 1985, in Stellar Redial Velocititw, should be of some significance for mod- Danish Natural Scimm Research Wn- IAU Colloq. No, 88, eds. A.G.O. Phllip, ds of the evolution of the Galactic thin cil, and the Carlsberg Foundation for D.W. Latham, L Oavls Press, Schenec- disk population. financial support. tady, p. 35. Why did Mazzel and Pigatto (1 988) M-, P., Plmtto, L: 1988, APtrran. m- derive ages a factor of three lowfrom phys. 193,148. their overshooting mcdels, when fhey Nlssen, P.E.: 1988, Astron. ~ys.I@@, Referencgs 346. considered their ages accurate to Andersen, J., Clausen. J.V., Gustafason, B., Wni,A: 1987, &&on. Astmphys. 180,49. -1 0 %7 Thdr determination of redden- Nordstrtim, B., VardenBerg, DA.: 1988, Sackmann, 1.4, Boothmyd, A.I., Fowler, ing h the Isochrone fit itself is a likely &tron, Astmphys. IW, 128. Wk. IsPo,& J. Sao, 727. reason: Their values of E(B-Vj are -0.16 Andersen. J., Nordstrbm, B., Clausen, J.V.: Vanden-, DA: 1985, Ap. J. Sup@. 68, mag larger than those observed directly, 1990, Ap J. 389, L33. 711.

The Oldest Stars A. ARDEBERG', H. LINDGREN~and I. LUNDSTR~M' '~undObservatory, Sweden; 2European -hem Observatory

Introduction the reasons for this preference, the gen- same Is definitely not true for the oldest erally high luminosity of hot stars has populations In the Galaxy. The major Traditionally, young stars haw been played a major role. As a rmult, our reason for our limited insights into the favourfte objects for Investigations of knowledge of the young populatlon in early generation of galactic stars is their galactic structure and dynamics. Among the Galaxy is relatively advanced. The low luminosltles and generally Incon-