2. The stellar models
As subdwarfs belong to population II, their initial helium content is close to the primeval value. Then stellar models depend only on age and metal content, assuming that the physical description is completely settled.
In old metal poor stars, elements enrichment seems to be always present (see e.g. Wheeler et al. 1989), though large variations exist from one star to the other. We have used in all our models the mixture of Allard (1996) with the -enrichment relative to the Sun, . Four initial metal contents, represented by the standard [Fe/H] observable, have been considered: . The initial total amount of helium, i.e. 3 He + 4 He , per mass is fixed to its primordial value assumed to be (Izotov et al. 1997). With respectively , X and Z as the iron, hydrogen and heavy elements mass ratio, we use the relation:
to relate the observed metallicity and metal content. Here for the solar mixture of Grevesse & Noels, (1993), and for the Allard's -enriched mixture.
The initial mass ratio X and Z are derived from the initial values of Y and [Fe/H] as given in Table 1.
Table 1. [Fe/H] and initial abundances, per mass unit, of hydrogen X and heavy elements Z.
To describe the lower part of the main-sequence we present here models of masses varying from 0.6 to evolving up to 10 Gy. Below , evolution is extremely slow, and above subdwarfs have left the main-sequence. As during the rapid pre main-sequence phase, the stellar models are almost fully mixed by convection, the microscopic diffusion is inefficient, and starting the computations at the homogeneous main-sequence stage is sufficient for our purpose.
© European Southern Observatory (ESO) 1999
Online publication: April 12, 1999