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198 9Apj. . .347. .835G the Astrophysical Journal, 347:835-848 .835G The Astrophysical Journal, 347:835-848,1989 December 15 © 1989. The American Astronomical Society. All rights reserved. Printed in U.S.A. .347. 9ApJ. 198 CARBON ISOTOPE RATIOS AND LITHIUM ABUNDANCES IN OPEN CLUSTER GIANTS K ALP ANA KrISHNASWAMY GiLROY1 Department of Astronomy and McDonald Observatory, University of Texas, Austin Received 1989 February 21 ; accepted 1989 June 13 ABSTRACT New high-resolution, high S/N spectra of CN lines at 8000  and Li lines at 6707  have been obtained for giants m about 20 Galactic open clusters and 12C/13C ratios and Li abundances determined for these stars. The ages of the clusters vary from about 50 million years to 5 billion years, and their turn-off masses vary from 1 M0 to about 6 M0. The ages and turn-off masses were determined by fitting theoretical isochrones to the cluster color-magnitude diagrams. Correlation of the isotope ratio and the Li abundances with the cluster turn-off masses indicates the following: (1) the 12C/13C ratio increases steeply with the turn-off mass until a mass of approximately 2.2 M0 when the ratio levels off abruptly to a value near 26; (2) older clusters with 12 13 turn-off masses lower than about 2.2 M0 in general exhibit C/ C ratios that are considerably lower than the theoretically predicted values while those with larger turn-off masses (M > 2.2 M0) show ratios close to standard predictions; and (3) no strong correlation exists between the Li abundances and the cluster turn-off masses, and the Li abundances in giants are, in general, lower than theoretically predicted values. Various theories to explain the observed abundance trends are discussed. Subject headings: clusters: open — stars: abundances — stars: evolution I. INTRODUCTION extremely difficult task in most field giants, especially the C, N, O abundances and 12C/13C ratios in red giants are Population I giants since these are spread over a fairly wide important indicators of nuclear processing that occurs inside a range of ages. star. As the star evolves up the giant branch, its convective An ideal way of getting around the problem of mass uncer- envelope expands inward and mixes approximately the outer tainty is to observe giants in open and globular clusters. Since 50% of the star by mass. The major elements to be mixed to the all the stars in the cluster are assumed to have been formed at surface are 13C and 14N, both products of the CN cycle which the same time, the cluster age can be determined by fitting occurs in the hydrogen shell outside the core. The convective theoretical isochrones to the color-magnitude diagrams, and mixing also transports the primordial 12C and fragile, light the cluster turn-off mass can be found by comparison with elements like Li, Be, and B, from the surface to the interior. The theoretical evolutionary tracks. This mass can then be used as net result of this so-called first dredge-up phase is to decrease a fair representation of the stellar mass on the giant branch. the surface 12C/13C and 12C/14N ratios and the abundances of This method, although not perfect, provides a fairly good esti- the light elements. Hence, a study of these quantities would mation of the giant mass. provide a valuable insight to the extent of convective mixing in A literature survey shows that there have been only a few red giants. recent studies of carbon isotope ratios and Li abundance in Studies of C, N, O and the 12C/13C and 12C/14N ratios in cluster giants. The Hyades giants studied by Tomkin, Luck, field giants have been undertaken by several authors, e.g., and Lambert (1976) for their carbon isotope ratio and by Tomkin, Lambert, and Luck (1975); Tomkin, Luck, and Lambert, Dominy, and Sivertsen (1980) for their Li abundnace Lambert (1976); Dearborn, Lambert, and Tomkin (1975); showed abundances that generally agreed with the theoretical Lambert and Ries (1977, 1981); and Kjaegaard et al (1982) for predictions. The 12C/13C ratio in giants in NGC 7789 was field Population I giants, Cottrell and Sneden (1986) for old obtained by Sneden and Pilachowski (1986), and the Li abun- disk giants, and Sneden, Pilachowski, and VandenBerg (1986) dance in this cluster was obtained by Pilachowski (1986). Their for field Population II giants. A comparison of these results results indicated that the isotope ratio was lower and the Li with standard theoretical predictions shows that in all cases, a abundance somewhat higher in these stars than the predicted considerable fraction of the stars exhibit carbon isotope ratios values. Li abundances in both M67 and NGC 752 were found that are much lower than the theoretical predictions. However, to be considerably lower than the theoretical predictions the theoretical predictions, both for the 12C/13C ratios and the (Pilachowski, Saha, and Hobbs 1988). Brown (1987) has Li abundances, are usually computed as a function of stellar published the 12C/13C ratios for two giants in M67, and he mass (see, e.g., Iben 1965, 1966, 1967 for the Li abundance and also obtains values lower than predicted. Hence, we see that Dearborn, Eggleton, and Schramm 1976 and Dearborn, one must go beyond standard stellar evolution theories to Tinsley, and Schramm 1978 for the 12C/13C ratios). Hence, explain the anomalous abundances observed in these giants. accurate comparison of observations with theory can be made The aim of this project was to determine the 12C/13C ratios only if the mass of the star is known fairly well. This is an and Li abundances in a comprehensive sample of late-type giants in clusters of varying ages and to correlate these abun- 1 Visiting Astronomer, Kitt Peak National Observatory, National Optical dances with the cluster turn-off masses. This is the first study of Astronomy Observatories, operated by the Association of Universities for this kind, and a comparison of these results with the theoretical Research in Astronomy, under contract with the National Science Founda- predictions should help provide a more realistic description of tion. the behavior of stellar mixing with mass. We chose to study the 835 © American Astronomical Society • Provided by the NASA Astrophysics Data System .835G 836 GILROY Vol. 347 .347. 12 13 . C/ C ratio rather than the C/N ratio since the former is, in have been determined by various authors for the open clusters general, not affected by uncertainties in model atmosphere in question (e.g., Sandage and Eggen 1969; Harris 1976; parameters, unlike the latter. This is because, unless the 12C Barbaro, Dallaporta, and Fabris 1969; Patenaude 1978; Van- 9ApJ. lines are saturated, the uncertainties in the atmosphere param- denBerg 1985; etc.), there is a fair amount of disagreement 198 eters affect both the 12C and 13C lines the same way, leaving among the various results. We therefore decided to carry out the isotope ratio unaffected. Hence the 12C/13C ratio can be an independent study of these parameters by fitting theoretical determined with greater accuracy than the C/N ratio. isochrones to the cluster color-magnitude diagram. There was one other important reason for undertaking this Of all the available theoretical evolutionary tracks and iso- project. Theorists over the past few years have presented a chrones, those published by VandenBerg (1985) provided the number of mechanisms to explain the observed low 12C/13C best fits to the open cluster color-magnitude diagrams. These ratios in field giants (e.g., Dearborn, Eggleton, and Schramm calculations are based in a He abundance, Y, of 0.25, and a, the 1976; Dearborn, Tinsley, and Schramm 1978; Sweigart and ratio of mixing length to pressure scale height of 1.6. The iso- Mengel 1979; Schatzman 1977; Bienaymé, Maeder, and chrones span a range of ages from 0.3 to 15 Gyr, and metal- Schatzman 1984). To confirm or reject these theories, it is licities from [Fe/H] = — 1.0-0.0. We used these isochrones for important to know the masses and evolutionary stages of the all our clusters except those younger than 0.3 Gyr. The iso- giants. Once again, this is not an easy task for the field giants, chrones of Barbaro, Dallaporta, and Fabris (1969) were used to and observations of cluster giants spanning a wide range of determine the ages and turn-off masses of the youngest clusters ages would help us test these theories more accurately. of our sample. The absolute magnitudes, B—V colors, and In §§ II and III we will briefly describe the selection criteria reddening were taken from the latest available photometry for and age determination for the cluster giants, and in § IV we each cluster. The best fitting isochrone gave the age of the discuss the observations and data reduction. Sections V and VI cluster, and the mass at the bluest point on the isochrone was deal with the analysis and discussion of the results, respec- the cluster turn-off mass. Table 2 gives the ages (t) and turn-off tively. masses (M/M0) determined for each cluster along with the reddening and distance moduli used to fit the isochrones. The II. SELECTION OF SAMPLE CLUSTER STARS errors were determined by the scatter in the color-magnitude diagrams, and the size of these errors depended on the ages and One of the main problems in observing stars in any cluster is turn-off masses. We therefore present in Table 2 the ratios At/t ascertaining their membership. Membership uncertainties arise and AM/Mq. Figure 1 shows sample isochrone fits for two of due to lack of good photometric and proper motion measure- the open clusters.
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