Cowley, A.J., Hutchings, I.B.: 1979, P.A.SP. 90,636. Debray, B., Dubout, R., L1ebaria, A., Petit, M.: 1988, Astron. Astrophys. (submitted). Feitzinger, J. V., Schlosser, W., Schmidt-Ka­ ler, Th., Winkler, C.: 1980, Astron. Astro­ + + phys. 84, 50. + . + Feitzinger, J. V., Isserstedt, J.: 1983, Astron. 4 + ~ + +. Astrophys. Supp. Ser., 51, 505. Henize, K. G.: 1956, Astrophys. J. Suppl. 2, 315. Heydari-Malayeri, M., Magain, P., Remy, M.: 1988, Astron. Astrophys. 201, L 41. Lortet, M. C., Heydari-Malayeri, M., Testor, 0 G.: 1986, in Luminous stars and associa­ 2 A Brey 70a tions in galaxies, Proceed. lAU Symp. 116, (l) I A 401. (l) + Lucke, P.B.: 1972, Ph.D. Thesis, University \J A of Washington. ::J ...... Lundström, 1., Stenholm, B.: 1984, Astron. C 0'1 Astrophys. Suppl. 58, 163. 0 + Morgan, D.H., Good, A.R.: 1987, ::::;E MNRAS. o Brey 73 224,435. 0 + Prevot-Burnichon, M. L., Prevot, L., Rebeirot, E., Rousseau, J., Martin, N.: 1981, Astron. + o Brey 71 Astrophys. 103, 83. Smith, L.F.: 1968, MNRAS. 140, 409. Stetson, P.B.: 1987, P.A.S.P. 99,191. Van der Hucht, K.A., Hidayat, B., Gunowan Admiranto, A., Supelli, K. R., Doom, C.: - 2 l.-----L_--"--_-'-----'_-'-_-'------''------'-_--'-_'--------'-_--'-_''-------L_--''-----' 1988 Preprint. -2 o 2 4 6 Walborn, N.R.: 1977, Astrophys. J. 215, 53. Magnitude Continuum Walborn, N. R.: 1980 (Private communica­ tion). Figure 6: Plot of the Hell relative magnitudes versus the continuum magnitudes of all stars of Weigelt, G., Baier, G.: 1985, Astron. Astro­ the frame (90 arcsec x 140 arcsec). The known W-R stars Brey 70a, Brey 71 and the Brey 73 phys. 150, L 18. association show a conspicuous Hell brightness excess. The stars belonging to the aggregate Westerlund, B. E.: 1974, Proceedings of the are represented by triangles. Most of the stars follow a close relationship v (A 4686 A) = 0.96 v first European Astronomical meeting in ()... 4794 A) + 0.34 and with a 0 rms of 0.22 magnitude after elimination of Brey 70a, Brey 71 and Athen, September 4-9, 1972, Volume 3. Brey 73. Star 12 is a faint star of the cluster, probably a red one which would need deep, good Springer, Berlin, Heldeberg, New York, seeing Band V photometry. p.39. Spectroscopic Identification of White Dwarfs in Galactic Clusters O. REIMERS, Hamburger Sternwarte, Universität Hamburg, F. R. Germany 0. KOESTER*, Oepartment ofPhysics andAstronomy, Louisiana State University, USA The End Products of Stellar fully as a white dwarf or undergoes a local SN 11 rate (e. g. Tammann, 1974) Evolution core collapse with a quasi instantane­ teils us that all stars more massive than ous energy release of - 1053 erg, visible 5 to 10 MG must become supernovae, The end products of the evolution of as a supernova explosion. and Kennicutt (1984) found that SN II's single stars are weil known: low mass Estimates of the maximum initial in Sc galaxies come from stars with stars leave white dwarf remnants, mas­ mass limit M for formation of white masses greater than 8 ± 1 MG' sive stars undergo a supernova explo­ WD dwarfs have been made by various Since the fate of intermediate mass sion with either a neutron star or a black techniques. stars is mainly determined by mass loss hole as a remnant. One possibility is to compare the in the red giant stage, the combination In a first approximation, it appears to supernova type I1 rate with the birth rate of stellar evolution tracks through the be only the initial mass which deter­ of massive stars in our galaxy. Since the red giant stages with empirical red giant mines whether a star ends its Iife peace- former rate is extremely uncertain and mass-Ioss rates also provides an esti­ can be observed only in external galax­ mate of MwD . The difficulty of this ap­ ies whereas the initial mass function can proach is that while mass-Ioss rates in be measured only in the solar neigh­ • Formerly at Institut für Theoretische Phy­ the normal red giant region are fairly weil sik und Sternwarte der Universität Kiel, F. R. bourhood, the constraints on SN 11 par­ known and can be parametrized by Germany ents are not stringent. An estimate of the semi-empirical interpolation formulae 47 .~ . .'0:' ...• .. •• • .. • .. .. •• " • • • ., .... • • • ' • .',: . .' '. ',f. ; '. • 0•• '. #.. • '. .. " , , • ... •• . , ..: ,: • . ' . ~' • " .' .. ' .. , ' : .. ' .. .~ ... ' " ,.. •• • .~~ " ..... t. • • ... • " . "~\.. ',: .... :...• '7'7.:' . .'. '.' .' . ,10. .. .,.. .. '. ,- '.• . " .. ,.' Figure 1: Central part of NGC 3532 (ESO Schmidt, 11I a-J+ UG 1; 75 min) with marked positions of candidate objects. (e. g. M- Ug . R, Reimers, 1975, 1987), ades. He found MWD = 4 MG' However, fields - in a number of intermediate age an extrapolation to advanced red giant with a turn-off mass of - 2 MG the Hy­ northern clusters. Based on their find­ phases (e. g. OH-IR stars) where most ades are not favourable for this method ings, they concluded MWD = 7 MG' mass is lost is certainly not allowed. On since according to the cluster luminosity However, the question remained the other hand, high mass-Ioss rates of function, their known six white dwarf whether the excess of blue objects in advanced red giants as observed in members must have had pragenitor the cluster fields were really white thermal GO lines or in OH maser lines masses only slightly higher than the dwarfs. Anticipating the results of our cannot easily be linked to stellar evolu­ turn-off mass. spectrascopic observations over the tion calculations, since for these stars years 1980-88 from both La Silla and masses and evolutionary stages are Galar Alto of all Romanishin and Angel's Faint Blue Objects in Open poorly known. candidates, it finally turned out that only Cluster Fields The only reliable method to determine 6 out of 17 WO candidates in the four Mwo seems to be the identification of A new attempt to determine MWD fram clusters NGG 2168, 2242, 2287 and white dwarf members of galactic clus­ cluster white dwarfs was made by 6633 are cluster members. This means ters with turn-off masses Mt :s MWD' Romanishin and Angel (1980) who that without time consuming spec­ The first attempt with this method was looked for a statistical excess of faint trophotometry of the faint 01 = 19 to 21) made by v. d. Heuvel (1975) far the Hy- blue objects - relative to comparison blue objects in the cluster fields, no safe 48 Confirmed white dwarf members of intermediate age clusters tions of WOs in cluster fields at such m faint magnitudes (-20 ) in the near fu­ 3 Cluster Age Turn-off White Te [10 K] V Mass Progenito ture. (Paper No.) logT Mass Dwarf [MG) Mass [MG) The Present Status NGC 2516 8.0 4.75 -1 30 19.6 0.95 7.2 How many white dwarf members of (11) -2 36 19.55 1.14 7.6 intermediate age clusters (turn-off -5 34 20.1 1.17 9.8 masses ;;:: 4 MG) are known at present after the spectroscopic observations 4 NGC 2451 ) 7.8 5.3 -6 31 16.75 0.6: 5.9 with the 3.6-m + lOS in the years (111) 1980-84, the new start with EFOSC on NGC 3532 8.3 3.6 -1 28 19.55 0.9 4.1 fainter objects in 1988, and a few nights M -5 28 19.2 0.6 3.9 on the northern clusters NGC 2168 and -6 29 19.9 0.9 4.1 NGC 6633 with the Calar Alto 3.5-m telescope? Altogether we have iden­ NGC 2168 8.0 4.75 -3 38 .20.25 0.7 5.9 tified 12 (possibly 14) WO members of (IV) -4 44 20.05 0.7 5.9 intermediate age clusters (Papers I-VI). 1 Four were from Romanishin and Angel's NGC 2287 8.25 3.9 ) -2 25 20.1 0.6 (I) -5 25 20.1 0.6 (1980) candidate list (two in both NGC 2287 and NGC 2168). Eight (ten) hot OA NGC 6405 7.9: 5.3: -1 3) 82 18.1 1.18 6.5: white dwarfs (the hottest has 82,000 K!) (VI) have been discovered by ourselves on 2 Pie/ades 7.9 5.3 LB 1497 0.85 6.2 ) the ESO Schmidt plates taken for that particular purpose. Only one WO, the ') According to Romanishin and Angel (1980). 2) Weidemann (1987). Pleiades member LB 1497 (e. g. Green­ 3) Cluster membership uncertain, WO is _1 0 from cluster centre. stein, 1974). was known to be member .) NGC 2451 contains two more probable WO members (Paper 111), if conrirmed they have high masses of an intermediate age open cluster be­ and high progenitor masses; : denotes uncertain .or preliminary values fore we started this project in 1980. ESO's superb equipment and dark sky has thus made possible an important conclusions are possible. The results of 30,000 K) are extremely rare in the gen­ contribution to an understanding of the the first two observing years have been eral field and projection onto a cluster is final stages of stellar evolution. described briefly by Koester (1982). therefore highly improbable, two criteria Our first observing run with EFOSC Parallel to this activity we asked for are applied: (i) the theoretically calcu­ and the 3.6-m telescope in April 1988 deep ESO red and UV Schmidt plates of lated cooling time, which depends on conducted by O. K. was highly success­ suitable southern intermediate age clus­ mass M and temperature Te must be ful. We identified 3 WO members of ters with turn-off masses around 5 MG' smaller than the cluster age and (ii) the NGC 3532.