A THEORETICAL APPROACH TO GLOBULAR-CLUSTER LOW MAIN-SEQUENCE STARS

This paper present new evolutionary computations extending toward the lower main sequence previous computations given for globular cluster s tars. We discuss the physical inputs with particular regard to the lim it of the adopted usual treatment of the stellar atmosphere, which all ows a tight correlation with actual stellar structures only for select ed ran of masses, depending on the assumed metallicity. Theoretical ev olutionary properties of Very Low Mass (VLM) stars are presented and d iscussed. We find that adopting new extended tabulations of radiative opacity, as computed according to Alexander & Fergusson (1994) procedu res and by relying on the Saumon & Chabrier (1992) equation of state ( EOS) our results allow a reasonable fitting of HST observation of meta l poor VLM objects in the galactic globular NGC 6397. The investigatio n has been extended to a sample of held VLM star with known parallaxes , showing again a satisfactory agreement with the distribution of meta l poor dwarfs and discussing the present theoretical scenario for sola r metallicity VLM. Selected evolutionary quantities are presented in s uitable 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-lu minosity relations for VLM objects for the various investigated metall icities closes the paper.