A theoretical approach to globular cluster low main sequence stars

This paper present new evolutionary computations extending toward the lower main sequence previous computa- tions 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 cor- relation 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 pre- sented and discussed. We nd that adopting new extended tabu- lationsofradiativeopacity,ascomputedaccordingtoAlexander & Fergusson (1994) procedures and by relying on the Saumon & Chabrier (1992) equation of state (EOS) our results allow a reasonable tting of HST observation of metal poor VLM ob- jects in the galactic globular NGC 6397. The investigation has been extended to a sample of eld VLM star with known par- allaxes, showing again a satisfactory agreement with the distri- bution of metal poor dwarfs and discussing the present theoret- ical 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 discus- sion including a comparison of mass-luminosity relations for VLM objects for the various investigated metallicities closes the paper.