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Astron. Astrophys. 317, 90-98 (1997)

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A theoretical approach to globular cluster low main sequence stars

D.R. Alexander 1, E. Brocato 2, S. Cassisi 2, 3, V. Castellani 2, 4, 5, F. Ciacio 6 and S. Degl'Innocenti 6, 7

1 Physics Department, Wichita State University, Wichita KS67260, USA
2 Osservatorio Astronomico di Collurania, Via M. Maggini, I-64100 Teramo, Italy
3 Dipartimento di Fisica, Università dell'Aquila, Via Vetoio, I-67100 L'Aquila, Italy
4 Dipartimento di Fisica, Università di Pisa, Piazza Torricelli 2, I-56100 Pisa, Italy
5 INFN Lab. Naz. Gran Sasso, I-67100 L'Aquila, Italy
6 INFN Sezione di Ferrara I-44100 Ferrara, Italy
7 Max-Planck Institut fur Astrophysics, K. Schwarzschild Str. 1, D-85740 Garching bei Munchen, Germany

Received 6 November 1995 / Accepted 21 May 1996


This paper present new evolutionary computations extending toward the lower main sequence previous computations given for globular cluster stars. We discuss the physical inputs with particular regard to the limit of the adopted usual treatment of the stellar atmosphere, which allows a tight correlation with actual stellar structures only for selected range of masses, depending on the assumed metallicity. Theoretical evolutionary properties of Very Low Mass (VLM) stars are presented and discussed. We find that adopting new extended tabulations of radiative opacity, as computed according to Alexander & Fergusson (1994) procedures and by relying on the Saumon & Chabrier (1992) equation of state (EOS) our results allow a reasonable fitting of HST observation of metal poor VLM objects in the galactic globular NGC 6397. The investigation has been extended to a sample of field VLM star with known parallaxes, showing again a satisfactory agreement with the distribution of metal poor dwarfs and discussing the present theoretical scenario for solar metallicity VLM. Selected evolutionary quantities are presented in suitable tables for metal poor objects (Y=0.23, Z=0.0001, 0.0003, 0.0006 and 0.001). A brief discussion including a comparison of mass-luminosity relations for VLM objects for the various investigated metallicities closes the paper.

Key words: stars: evolution — stars: Hertzsprung-Russel diagram — stars: population II — globular clusters: general — stars: low-mass, subdwarfs

Send offprint requests to: V. Castellani, Dipartimento di Fisica Universita' di Pisa, Piazza Torricelli 2, I-56100 Pisa, Italy

© European Southern Observatory (ESO) 1997