Grid of stellar models at Z=0.0004

A new grid of stellar models at very low metallicity (1/50 solar) has been calculated by one of us for various applications (see de Mello et al. 1998, Stasinska & Schaerer 1999). The full grid of stellar tracks from 0.8 to 150 M_ M$_{\odot}$ is briefly summarised here.

The input physics is identical to that of Meynet et al. (1994, paper V). This includes in particular the adoption of ``high'' mass loss rates, i.e. M_ M$_{\odot}$/yr for WNL stars and twice the mass loss rates of de Jager et al. (1988) for stars with _. Otherwise we follow the prescription of paper I. For consistence with the earlier grids the initial composition is calculated as in paper I: X=0.7584, Y=0.2412, Z=0.0004.

The HRD with the calculated tracks is shown in Fig. A1 Hatched areas indicate regions of slow nuclear burning phases. The H, He, and C burning lifetimes are given in Table . Due to numerical instabilities the 85 M_ M$_{\odot}$ model could not be fully evolved to the end of He burning. The remaining He-burning lifetime as well as the duration of C-burning were estimated by interpolation between the two adjacent tracks. For similar reasons of the 20 M_ M$_{\odot}$ model had also to be estimated.

The mass limit for the formation of WR stars, , at Z=0.0004 from the present tracks is 85 M_ M$_{\odot}$, as already derived in de Mello et al. (1998). Other properties of the Z=0.0004 models can readily be derived from the detailed data included in the present database.

Figure: Theoretical HR diagram for the ensemble of the calculated models for metallicity Z=0.0004 with an overshooting parameter . The slow phases of nuclear burning are indicated by hatched areas. The track corresponds to the model without overshooting

Table: Lifetimes in nuclear phases (in units of yr) for