Your thesaurus codes are: 10.15.2, individual: NGC 2099, 08.07.2, 8.02.4, 03.20.7 AND ASTROPHYSICS 30.10.1995 Red giants in open clusters. V. NGC 2099?,?? J.-C. Mermilliod 1, G. Huestamendia 2, G. del Rio 2, and M. Mayor 3 1 Institut d'astronomie de l'universite de Lausanne, CH-1290 Chavannes-des-Bois, Switzerland 2 Observatorio Astronomico de Madrid (IGN), Alfonso XII, 3, 28014 Madrid, Spain 3 Observatoire de Geneve, CH-1290 Sauverny, Switzerland Received: July 5; accepted: July 5 Abstract. 474 radial-velocity observations and new U BV photoelectric data for 55 and 20 red giants respectively, in the eld of the intermediate-age open cluster NGC 2099 are presented and analysed for membership and duplicity. The membership of 35 red giants has been conrmed, 16 spectroscopic binaries have been discovered and 11 orbits have been determined. The cluster binaryfrequency lower limit is slightly above the average (9/35 = 26%). The radial distribution of the red giants shows a sharp limit between members and non-members. No mass segregation is observed between the binary and single red giants. The latest isochrones from the Geneva and Padova groups reproduce very well the observed morphology in the colour-magnitude diagram for log t = 8.65 and z = 0.02, with (m? M) = 11.50 and E(B? V ) = 0.29. Key words: cluster: open { individual: NGC 2099 { radial velocity { star: red giant { binary: spectroscopic { photometry: UBV 1. Introduction This fth paper in a series devoted to the study of red giants in open clusters by means of accurate radial-velocity data presents CORAVEL observations and new U BV photoeletric data in the intermediate-age and rich open cluster NGC 2099. Earlier papers discussed 93 red giants in the ve intermediate-age clusters NGC 2360, 2423, 5822, 6811 and IC 4756 (Mermilliod & Mayor 1990, paper III) and Send oprint requests to: J.-C. Mermilliod? Based on observations made at the Haute-Provence Observatory (France) and the Observatorio Astronomico de Madrid (IGN) at Calar Alto (Spain)?? The individual radial velocities are available in electronic form: see the Editorial in A&A 1994, Vol. 280, no. 3, p. E1 dealt with 35 stars in the two older clusters NGC 3680 and IC 4651 (Mermilliod et al. 1995, paper IV). Our approach is to identify non-member stars and discover spectroscopic binaries through accurate, long-term CORAVEL radial-velocity observations. In order to study the distribution of the red giants in the colour-magnitude diagram, we also present new photometric data in the Geneva or U BV photometric systems. NGC 2099 = C0549+325 ( = 5 h 49:1, = +32 32 0, B1950) is an interesting, populous intermediate-age open cluster which has not been much observed since the 1960s. The colour-magnitude diagram based on the data from major photographic U BV photometric studies (Hoag et al. 1961; Brosterhus 1963; West 1967) shows a rather broad main sequence and the published proper motions (Meurers 1960; Jeerys 1962; Upgren 1966) do not provide a clear separation of the cluster members from the eld stars. Robin (1982) obtained U BV electronographic data in an outer eld of the cluster. The cluster parameters from Lynga 's (1987) fth catalogue are m-m = 11.63, E(B-V) = 0.31 and log t = 8.30. NGC 2099 presents a well populated clump of red giants which makes it an interesting object to determine the red giant membership, search for spectroscopic binaries and try to determine the structure of the clump. Glushkova & Rastorguev (1991) published 14 radial-velocity observations of 9 red giants in NGC 2099 and their results are in good agreement with those presented here. 2. Observations 2.1. The sample denition Red stars brighter than V = 12. and redder than B? V = 0.50 were selected for radial-velocity observation, mostly from the U BV photographic data. The colour-magnitude diagram based on these data shows a prominent clump which contains most programme stars. The rather blue
ple a few stars that are supposed to be composite binaries because they fall more or less in the middle of the Hertzsprung gap. The survey made by van Zeipel & Lindgren (1921) extends a little further out from the cluster centre than the photographic surveys and candidate red giants were selected from the visual magnitudes (m vis ) and colours (C) from their paper. The sample contains a total of 55 stars. The numerous identications attributed to the red giants by eight dierent authors are summarized in Table 1. The numbering system of van Zeipel & Lindgren (1921) has been adopted in this study and is presented in column 1 (vz). The successive columns 2 to 8 contain the star numbers attributed by \N": Nordlund (1909), "G": Giebeler (1914), "J": Joy (1915), "M": Meurers (1960), "Bros": Brosterhus (1963), "Bron": Bronnikova (1958) and "West": West (1967). 2.2. Radial-velocity measurements As part of the general programme relating to evolved stars in open clusters, at least 5 radial velocity measurements were obtained for each member star with the radial velocity scanner CORAVEL (Baranne et al. 1979) installed on the 1m Swiss telescope at the Haute Provence Observatory (OHP, France) from 1978 to 1993. As a rule integration times range from 10 to 20 minutes depending on the night quality and seeing. The radial velocities were corrected for zero-point dierences to place them in the system dened by the southern CORAVEL. The 474 individual observations are available in electronic form from the Strasbourg anonymous ftp account (see the Editorial in A&A 280, no 3, 1994). The mean radial velocities are displayed in Table 3 and columns 6 to 11 contain successively the mean radial velocities and errors in [km s?1 ], the number of measurements (N), the time span (T) of the observations [days], the probability P( 2 ) that the mean error merely reects random observational errors for a constant star, and remarks on binarity and membership. 2.3. Photometric data During an observing run in November 1988 we obtained U BV photoelectric observations for 20 red giants with a single channel pulse counting mode photoelectric photometer attached to the 1.52m reector of the Observatorio Astronomico de Madrid (IGN) at Calar Alto (Spain). The dry-ice cooled photomultiplier and lter combinations used were described by Huestamendia et al. (1991). Standard stars were taken from the list of Neckel & Chini (1980). Counting errors were kept under the 1% level. The reduction procedure took into account possible dependence of the residuals on observing time, azimuth, hour angle and colour, and applied, where necessary, the corresponding corrections. Standard-star rms were below 0.02 magnitude. The new photometric data, mostly based Table 1. Cross-identications vz N G J M Bros Bron West 4 440 632 135 341 4461 230 4-1-10 7 497 655 165 390 4476 277 1-1-03 9 484 647 159 379 266 A 12 506 659 169 399 4551 283 1-1-12 16 463 642 149 364 4457 247 3-1-12 24 485 648 160 380 267 2-1-16 31 452 637 141 352 5470 240 3-2-10 34 428 627 127 328 219 3-2-11 49 352 592 97 269 5451 166 3-2-22 64 400 617 116 309 4463 204 F 67 446 872 139 348 4471 236 C 92 685 456 107 579 690 203 463 5550 330 N 117 500 434 167 395 5466 280 2-2-18 120 481 432 156 377 5467 261 2-2-20 148 339 392 94 257 5354 159 3-4-14 149 362 398 100 278 5457 173 3-3-20 172 300 577 81 226 4362 135 4-3- 6 255 546 445 188 434 4100 307 2-5-88 277 281 372 76 211 4300 123 3-4-18 350 691 726 243 557 5662 399 J 391 306 381 83 233 6355 140 3-5-45 401 233 366 62 166 4500 96 3-5-66 412 199 546 54 145 78 3-5-92 439 240 819 65 171 3351 99 4-5-51 454 391 1005 112 2453 195 4-5-87 474 670 1040 232 539 2653 386 1-6-110 485 725 744 254 588 4752 415 Q 488 728 745 255 591 416 K 508 630 274 222 505 6655 362 2-6-94 563 135 339 36 102 5154 46 3-6-208 599 392 1006 114 2451 198 4-6-203 608 1019 1505 1-6-28 656 658 284 229 530 7655 378 2-6-144 685 109 210 30 81 3700 36 3-6-163 716 238 988 64 168 2253 98 4-6-145 725 524 1117 179 412 600 296 748 786 934 633 3852 448 1-6-169 782 460 150 147 360 9453 838 146 976 42 111 1251 55 865 808 937 277 651 6900 460 919 81 206 22 62 7051 24 955 421 1192 966 740 1134 259 601 753 421 974 1159 1006 798 302 275 646 8854 456 1040 128 118 34 95 9155 1159 838 316 288 691 7952 480 1192 15 49 9052 1241 1159 88 1376 5 1377 6 3000 1505 1666 1805
4 of table 3 (source #1). The other sources of U BV photoelectric photometry are: (2) Coleman (1982), (3) West (1967), (4) Jennens & Helfer (1975), (5) Brosterhus (1963). Sources (6) and (7) refer to the photographic data from West (1967) and Brosterhus (1963) respectively, corrected for the mean dierences given in Table 2 computed in the sense our photometry minus the photographic data. For the stars without U BV data, only the magnitudes (m vis ) from van Zeipel & Lindgren (1921) have been tabulated (source 8). This concerns mostly non-members and it does not aect the aspect of the colour-magnitude diagram. Table 2. Mean dierences UBV pe { pg V (B-V) (U-B) N* Source Mean di -0.033 0.093-0.084 15 Brosterhus.043.044.034 Mean di -0.001-0.049 0.078 16 West.064.042.079 elements are given in Table 4. A few stars deserve special comments. vz 92 has a close companion and it is dicult to know where the automatic guiding sets the entrance slit. The scatter in the data may be simply due to the eect of the companion. vz 255 has a wide dip and the individual errors are large. Twenty-two observations were done but no period could be found. The last, better integrated, observations do not show the same large scatter. vz 1006 and 1866, both non-members, have three velocities in very close agreement and one observation about 2 km s?1 apart. Their binary nature seems very unlikely, although P( 2 ) = 0.000 and is noted \SB??" in the remarks. vz 412 presents a very small mass function (0.08) and four of the ve binary members with an orbit have a value of M 2min / M 1 less than 0.5. The problem of the distribution of the secondary mass as a function of the primary mass will be discussed in a forthcoming paper collecting the published orbits and additional material to be published. 3. Results 3.1. Spectroscopic binaries The membership of a spectroscopic binary is dicult to determine until an orbit has been obtained or the observations are numerous enough to decide whether the mean velocity and the range of variation are compatible with membership. The presence of 20 to 25% of spectroscopic binaries among the red giants in open clusters requires numerous observations to determine the orbits. In NGC 2099, fourteen spectroscopic binaries, of which only 9 are really cluster members were discovered. Eleven orbits, some of them being still preliminary, have been determined. Less observations were done on the non-member binaries when their non-membership was recognized. The orbital elements are given in Table 4. Some errors may seem rather large. They result partly from the length of some periods and partly from the somewhat small amplitudes, for vz 412, 685, 966, for example. The radialvelocity curves for the ve member binaries (vz 49, 149, 412, 485, 966) are displayed in Figs. 1, 2, 3, 4, and 5. Two additional stars (vz 12 and 748) show clear radial-velocity variations, but the periods will be very long, i.e. much greater than 5000 d, and quite eccentric. The radial velocities are plotted in function of the Julian dates of observations in Fig. 6 and 7 respectively. The resulting binary frequency is 26% (= 9/35). Seven spectroscopic binaries were discovered among the non-member stars, and six orbits determined. The Fig. 1. Radial-velocity curve for vz 49. 3.2. Mean radial velocity and membership The distribution of the mean radial velocities for the 55 stars observed (Fig. 8) shows a pronounced maximum around 7.5 km s?1 and a clear separation of the members and non-members. Thirty-ve stars have been considered as cluster members and the others are marked with NM in Table 3. The mean velocity for NGC 2099 based on 30 non-variable stars and spectrocopic binaries with an orbit is 7.68 0.17 km s?1. The r.m.s dispersion around the mean is 0.92 km s?1. This high value is nearly twice that given for M67 (0.48 km s?1 ) by Mathieu (1985). It may
Table 3. New UBV photometry and radial velocity observations vz V B-V U-B S V r N T P( 2 ) Rem 4 11.54 1.18 0.86 1 8.28 0.28 5 4371 0.022 7 11.38 1.20 0.91 1 8.26 0.16 5 4370 0.988 9 9.19 1.66 1.30 1 6.92 0.19 6 4778 0.031 12 11.70 1.20 0.90 1 6.88 1.37 10 4755 0.000 SB 16 11.24 1.21 0.95 1 6.69 0.15 5 4372 0.472 24 11.57 1.19 0.93 7 8.33 0.15 6 4314 0.595 31 11.47 1.11 0.96 6 6.63 0.18 5 4369 0.364 34 11.07 1.18 1.12 7 6.32 0.16 5 4369 0.667 49 11.53 1.21 0.90 1 8.00 0.09 25 3692 0.000 SB, orbit 64 11.15 1.23 0.95 2 7.37 0.17 5 4370 0.294 67 11.04 1.20 0.88 2 7.53 0.16 5 4372 0.987 92 12.42 8 6.04 1.63 4 2918 0.000 107 11.31 1.26 1.04 1 6.78 0.23 5 4370 0.046 117 11.58 1.21 0.91 1 8.25 0.29 6 4698 0.000 SB 120 11.56 1.12 0.78 1 7.41 0.22 5 4372 0.100 148 11.09 1.28 1.06 1 8.21 0.19 5 4371 0.230 149 11.12 1.14 1.13 6 6.10 0.09 24 4727 0.000 SB, orbit 172 11.42 1.19 0.89 1 7.54 0.30 5 4373 0.007 255 10.92 0.63 0.43 1 6.65 0.65 22 3334 0.000 277 10.31 1.45 1.41 1 9.66 0.12 15 4727 0.003 350 10.32 1.50 1.50 2 8.07 0.21 5 3633 0.032 391 10.91 1.48 1.47 1 7.46 0.14 5 3652 0.928 401 11.35 1.23 1.00 1 8.61 0.19 5 4371 0.182 412 11.02 1.10 0.66 7 7.63 0.10 18 4032 0.000 SB, orbit 439 11.38 1.05 0.70 1 7.36 0.43 6 4033 0.000 SB 454 11.16 1.26 1.03 1 8.25 0.16 5 3649 0.354 485 10.23 1.44 1.28 4 8.64 0.10 27 3694 0.000 SB, orbit 488 11.16 1.28 0.85 3 7.79 0.16 6 3652 0.155 508 10.98 1.28 2 8.42 0.17 5 3723 0.311 563 11.68 1.02 0.59 6 0.31 0.17 7 3653 0.176 NM 599 11.09 1.85 2.10 1 21.03 0.19 19 2941 0.000 SB, orbit, NM 656 11.45 1.46 2 31.94 0.19 4 2201 0.220 NM 685 10.63 1.48 1.64 4 14.97 0.12 15 3653 0.000 SB, orbit, NM 716 11.61 1.17 0.84 1 7.15 0.21 5 3652 0.223 725 10.89 1.21 1.01 5 5.61 0.27 5 3651 0.007 748 11.72 1.10 2 2.54 1.19 22 4032 0.000 SB 782 10.28 1.10 0.70 6-4.01 0.11 18 3012 0.000 SB, orbit, NM 838 11.79 1.21 0.83 6 20.74 0.21 19 3262 0.000 SB, orbit, NM 865 11.49 1.50 1.57 5-4.46 0.10 20 4007 0.000 SB, orbit, NM 919 11.84 1.22 0.81 6-8.02 0.19 4 2202 0.296 NM 966 11.83 1.51 1.28 6 9.51 0.09 14 4032 0.000 SB, orbit 1006 10.69 1.66 1.75 6 21.05 0.38 4 2201 0.000 SB??, NM 1040 11.51 1.53 1.60 6-13.23 0.15 4 2202 0.464 NM 1159 10.91 0.92 0.40 6 26.42 0.18 4 2201 0.898 NM 1192 11.67 1.21 0.73 6 8.06 0.15 5 3640 0.675 1376 11.35 8 25.30 0.21 4 2194 0.196 NM 1377 11.71 8-22.49 0.28 4 2187 0.017 NM 1505 10.61 8-17.66 0.35 11 3640 0.000 SB, orbit, NM 1666 11.88 8-3.02 0.59 6 3989 0.000 NM 1805 11.60 8-13.25 0.28 4 2187 0.032 NM 1846 11.50 8 7.03 0.17 5 3633 0.223 1859 10.15 8-42.09 0.18 4 2195 0.224 NM 1866 9.50 8-23.00 0.52 4 2195 0.000 SB??, NM 2080 10.85 8-11.65 0.23 4 2192 0.121 NM 2084 10.41 8 14.83 0.52 4 2192 0.035 NM
Table 4. Orbital elements of eleven spectroscopic binaries in NGC 2099 vz P T e! K f(m) a sini M2min (O? C) n obs [d] [HJD] [km s?1 ] [ ] [km s?1 ] [M ] [Gm] [M ] 49 162.25 2448764.3 8.00 0.39 240.5 14.58 0.041 29.9 0.81 0.41 25 0.05.8.09.01 1.4.19.002.5 149 918.9 2447923. 6.15 0.13 119.0 4.94 0.0112 61.8 0.49 0.35 24 2.9 23..07.02 9.6.10.0008 1.6 412 261.3 2448664. 7.63 0.19 93. 1.42 0.00007 5.0 0.08 0.38 18 1.1 23..10.10 33..16.00003.7 485 455.46 2448321.5 8.64 0.168 46.1 16.1 0.190 99.5 1.52 0.52 27.29 3.8.10.009 3.5.2.006 1.1 599 168.88 2448871. 21.03 0.05 81. 11.69 0.028 27.1 0.77 19.12 12..19.02 27..30.002.7 685 1670. 2448427. 14.97 0.14 251. 2.84 0.0039 64.6 0.39 15 25. 106..12.05 24..15.0008 4.8 782 886.2 2449113.1-4.01 0.223 211.0 16.68 0.395 198.1 0.45 18 1.0 7.0.11.008 2.9.16.014 2.6 838 1558. 2448463. 20.74 0.33 122.8 10.28 0.14 207.8 0.81 19 11. 32..21.03 7.1.31.02 9.9 865 433.10 2448725. -4.46 0.04 252. 9.52 0.039 56.7 0.41 20.44 21..10.02 18..14.002.9 966 3079. 2448493. 9.51 0.39 32.3 3.49 0.011 135.9 0.49 0.31 14 33. 38..09.04 5.5.13.002 8.8 1505 2424. 2448640.9-17.66 0.81 255.2 18.63 0.33 364. 0.36 11 25. 7.8.35 (xed) 3.6.61.04 15. Fig. 2. Radial-velocity curve for vz 149. Fig. 3. Radial-velocity curve for vz 412. be due to yet undiscovered binaries with long periods and low amplitudes, but it certainly results for a large fraction from the true internal velocity dispersion since NGC 2099 is signicantly more massive than most clusters we have studied in previous papers. The mass for NGC 2099 published by Bruch & Sanders (1983) (531 M ) is grossly underestimated. The upper main-sequence stars have a mass around 2.5-3 M and their estimated mass would imply that there are only about 200 stars in NGC 2099, which is probably too small by a factor of ten.
Fig. 4. Radial-velocity curve for vz 485. Fig. 6. Radial-velocity curve for vz 12. Fig. 5. Radial-velocity curve for vz 966. Fig. 7. Radial-velocity curve for vz 748. 3.3. Radial distribution of the red giants Artjukhina & Kholopov (1964) studied the structure of the open cluster NGC 2099 by the star count method. They obtained a nucleus radius of 10 0, an inner corona radius of 30 0 and an outer corona extending out to 100 0. The area covered by the survey of van Zeipel & Lindgren (1921) is 44 0 x44 0. It therefore does not extend far enough to completely include the inner corona of the cluster. The area distribution of the red giant members (lled circles) and non-member stars (open circles) is displayed in Fig. 9. The scale is in minutes of arc. Thirty-one red giant members are included in a distance of 10 0 from the centre, and 34 are closer than 12 0. Most red giants are therefore contained in the cluster nucleus and the present sample probably contains a very large fraction of the red giant population of the cluster. NGC 2099 was included in a search for mass segregation in clusters of various ages made by Raboud & Mermilliod (1994). It belongs to the younger group, and shows no dierence in the radial distribution of binaries and single member stars. 4. The colour-magnitude diagram The second aim of this work on red giants in open clusters is to study the morphology of the red giant locus in the colour-magnitude diagram to test the predictions of stellar evolutionary models. The colour-magnitude diagram plotted in Fig. 10 represents a large improvement
and turno region. From the (U? B; B? V ) diagram, a colour excess E(B? V ) = 0.29 has been determined and ZAMS tting gave a distance modulus m? M = 11.50. The V magnitudes and (B? V ) colours of the red giants have been corrected by -0.03 and +0.03 mag. respectively to take into account the dierential reddening of red stars with respect to main-sequence stars. vz 391 (V = 10.88, B-V = 1.51) falls slightly to the red of the other stars and would also lie to the right of the isochrone when adjusted to t the clump position. Its radial velocity is very close to the cluster mean velocity and the U BV photoelectric data show also a very good agreement between the three sources. Therefore the observational data and evidence for membership are not questionable. Fig. 8. Radial-velocity distribution of the 55 stars observed. An isochrone from Schaller et al. (1992) with log t = 8.65 has been plotted over the observed points (Fig. 10). This age reproduces quite well the upper main-sequence morphology. The second sequence is the same isochrone moved upward by 0.75 mag. to simulate the binary limit. The two curves dene the main-sequence envelope and give a very good t to the observed narrowing. The predicted position of the red giants for the core-helium burning phase is slightly too red as compared with the observed points. However the observed red giant distribution is compatible with the expected distribution of the stars during the helium-burning phase, on the left part of the loop, although no structure can be discerned. Two isochrones from Bertelli et al. (1994) for z = 0.02 and log t = 8.60 and 8.70 have been plotted over the same colour-magnitude diagram (Fig. 11). The isochrones also reproduce quite well the upper main-sequence morphology. The isochrone for log t = 8.60 nicely predicts the magnitude of the clump, but is also slightly too red. Fig. 9. Projected distribution of the members and non-members. North is up and East is left. The scale unit is [arcmin]. with respect to the diagrams based on photographic data, but still shows some scatter. The main reason is probably related to the diculty of properly measuring the stars in such a crowded eld with a high sky background due to the fainter stars. The various sources of U BV photoelectric data present a generally good agreement, but dierences up to 0.05 mag. nevertheless exist. The existing photographic data for NGC 2099 are not precise enough to really constrain the ZAMS and isochrone tting. Furthermore the red giant B-V colours show signicant dierences that let them fall quite outside from the isochrone. Therefore the isochrones have been tted to the available photoelectric data, which, in spite of The isochrones from both groups are very similar on the main sequence, but closer examination of the red giant region shows some dierences in the detailed morphology of the loop representing the helium-burning phase. For example, the relative position of the bluer part of the loop with respect to the bottom of the red giant branch or the relative position of the faintest point after the red giant tip with respect to the termination of the main sequence are dierent. Models from the Padova group (Fig. 11) seem to reproduce better the observed characteristics. Unfortunately, there are few stars in the subgiant portion of the diagram to help deciding which grid of models oers the more appropriate description. Either the red giant tracks need a little correction to reproduce the observed red giant position in the colour magnitude diagram, or the metallicity of the cluster is slightly less than solar and the red giant colours are bluer. However NGC 2099 is not decient enough to justify the use of isochrones with z = 0.008.
Fig. 10. Colour-magnitude diagram with the stars in NGC 2099 measured photoelectrically. Filled squares stand for cluster members, and open square for non-members. The isochrone is from Schaller et al. (1992), for z = 0.02 and log t = 8.65. The second, incomplete, curve is the same isochrone displaced by 0.75 mag. to mark the binary limit. The adopted cluster parameters are: m-m = 11.50 and E(B-V) = 0.29. Note the very good agreement of the isochrone with the main-sequence morphology. The observations of CORAVEL radial velocities and new U BV photoelectric photometry of red giants in the eld of NGC 2099 have permitted to perform a clear selection of 35 certain members and to discover 16 spectroscopic binaries, among which 9 only are cluster members. Eleven orbits have already been determined. The comparisons of the observed morphology of the red giants in the colour-magnitude diagram with the theoretical isochrones for z=0.02 show a good agreement with isochrone for log t = 8.65 from Schaller et al. (1992) and for log t = 8.60 and 8.70 from Bertelli et al. (1994). The cluster parameters derived from the t (m-m = 11.50 and E(B-V) = 0.29) are close to the values given by Lynga in the fth edition of his catalogue (Lynga 1987). This cluster is very interesting for comparison with evolutionary models as well as for studying the structure of populous open clusters and the eect of mass segregation. It would therefore be important to get well calibrated CCD photometry to improve the main sequence denition. That resulting from the photographic photometry is much too broad to provide constraints strong enough to test the models. Acknowledgements. We are grateful to the referee, Dr G. Bertelli, for his valuable remarks on the evaluation of the colour-magnitude diagram. The work of JCM and MM has been supported by continuous grants from the Swiss National Foundation for Scientic Research (FNRS). GH and GR have been partially supported by Spanish Grant DGICYT PB92-0839. Fig. 11. Same colour-magnitude diagram as in Fig. 10. Two isochrones from Bertelli et al. (1994), for z = 0.02, log t = 8.60 and 8.70. The adopted cluster parameters are: m-m = 11.50 and E(B-V) = 0.29. Symbols as in Fig. 10. References Artjukhina N.M., Kholopov P.N. 1964, Soviet AJ 7, 840 Baranne A., Mayor M., Poncet J.-L. 1979, Vistas in Astron. 23, 279 Bertelli G., Bressan A., Chiosi C., Fagotto F., Nasi E. 1994, A&AS 106, 275 Bronnikova N.M. 1958, Tr. Glavn. Astr. Obs. Pulkove 72, 77 Brosterhus E. 1963, Astron. Ahb. Hamb. Sternw. VII no 2 Bruch A., Sanders W.L. 1983, A&A 121, 237 Coleman L.A. 1982, AJ 87, 369 Giebeler H. 1914, Vero. Univ. Sternw. Bonn no 12 Glushkova E.V., Rastorguev A.S. 1991, Sov. Astr. Lett. 17, 13 Hoag A.A., Johnson H.L., Iriarte B., Mitchell R.I., Hallam K.L., Sharpless S. 1961, Publ. US. Nav. Obs. XVII, 347, part VII Huebner W.F., Merts A.L., Magee N.H., Argo M.F. 1977, Los Alamos Scientic Laboratory Reports LA-6760-M Huestamendia G., del Rio G., Mermilliod J.-C. 1991, A&AS 87, 153 Iglesias C.A., Rogers F.J., Wilson B.G. 1992, ApJ 397, 717 Jeerys III W.H. 1962, AJ 67, 532 Jennens P.A., Helfer H.L. 1975, MNRAS 172, 681 Joy A.H. 1915, AJ.29, 101 Lynga G. 1987, Fifth catalogue of cluster parameters (Strasbourg)
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