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Open Clusters Under the Microscope 6 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.
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