Context. In many astrophysical contexts, the helium content of stars may differ significantly from those usually assumed in evolutionary calculations. Aims. In order to improve upon this situation, we have computed tracks and isochrones in the range of initial masses 0.15−20 M for a grid of 39 chemical compositions with the metal content Z between 0.0001 and 0.070 and helium content Y between 0.23 and 0.46. Methods. The Padova stellar evolution code has been implemented with updated physics. New synthetic TP-AGB models allow the extension of stellar models and isochrones until the end of the thermal pulses along the AGB. Software tools for the bidimensional interpolation (in Y and Z) of the tracks have been tuned. Results. This first paper presents tracks for low mass stars (from 0.15 to 2.5 M ) with scaled-solar abundances and the corresponding isochrones from very old ages down to about 1 Gyr. Conclusions. Tracks and isochrones are made available in tabular form for the adopted grid of chemical compositions in the plane Z-Y. As soon as possible an interactive web interface will allow users to obtain isochrones of any chemical composition and also simulated stellar populations with different Y(Z) helium-to-metal enrichment laws.

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We extend our theoretical computations for low-mass stars to intermediate-mass and massive stars, for which few databases exist in the literature. Evolutionary tracks and isochrones are computed for initial masses 2.50−20 M for a grid of 37 chemical compositions with metal content Z between 0.0001 and 0.070 and helium content Y between 0.23 and 0.40 to enable users to obtain isochrones for ages as young as about 10 7 years and to simulate stellar populations with different helium-to-metal enrichment laws. The Padova stellar evolution code is identical to that used in the first paper of this series. Synthetic TP-AGB models allow stellar tracks and isochrones to be extended until the end of the thermal pulses along the AGB. We provide software tools for the bidimensional interpolation (in Y and Z) of the isochrones from very old ages down to about 10 7 years. This lower limit depends on chemical composition. The extension of the blue loops and the instability strip of Cepheid stars are compared and the Cepheid mass-discrepancy is discussed. The location of red supergiants in the H-R diagram is in good agreement with the evolutionary tracks for masses from 10 to 20 M . Tracks and isochrones are available in tabular form for the adopted grid of chemical compositions in the extended plane Z − Y in three photometric systems. An interactive web interface allows users to obtain isochrones of any chemical composition inside the provided Z − Y range and also to simulate stellar populations with different Y(Z) helium-to-metal enrichment laws.

Abstract. -We present a large grid of stellar evolutionary models with the initial chemical composition [Z = 0.0001, Y = 0.23]. These tracks are conceived to extend the grid of stellar models described in the previous papers of this series, and are computed with the new radiative opacities by Iglesias et al. (1992) and convective overshoot. The tracks span the range of initial masses from 0.6 M to 100 M , and extend from the zero age main sequence (ZAMS) till very advanced evolutionary phases. Specifically, low-and intermediate-mass stars are followed till the beginning of the thermally pulsing regime of the asymptotic red giant branch phase (TP-AGB), while massive stars are followed till the core C-ignition. With respect to previous papers of this series, these models incorporate a number of small modifications in the input physics, particularly on the equation of state, which now incorporates the effect of Coulomb interactions between charged particles. The effect of these modifications is discussed. The corresponding theoretical isochrones are presented.

This paper represents a collective effort to provide an extensive electronic database useful for the interpretation of the spectra and evolution of galaxies. A broad variety of empirical and theoretical data are discussed here, and the data are made fully available in the AAS CD-ROM Series, Vo. 7. Several empirical stellar libraries are part of this database. They cover the ultraviolet spectral range observed with IUE, optical data from different ground-based telescopes, and ground-based infrared data. Spectral type coverage depends on the wavelength, but it is mostly complete for types O and M and luminosity classes V to I. A large metallicity range is covered as well. Theoretical libraries of selected spectral indices of cool stars and of stellar continuum fluxes in the temperature range 2000 K to 50,000 K, as well as Wolf-Rayet energy distributions are presented. Several libraries of star clusters and early-type galaxies have been selected for this database. We discuss an extensive set of empirical spectra templates covering the wavelength region from 1200 - 9800 A, as well as narrow-band line indices in a large number of passbands. Bench-mark spectra of nearby galaxies for model tests are included as well. We compiled numerous evolutionary models and isochrones for stars of all mass ranges of interest, wide metallicity range, and for all evolutionary phases, including the pre-main-sequence phase. The majority of the models have been computed by the Geneva and Padova groups. Evolutionary synthesis models computed by several independent groups are made available. They can be applied to old and young systems, and are optimized with respect to different aspects of input physics. The model predictions include stellar (colors, magnitudes, absorption features) and nebular (emission-line fluxes) properties. Finally, we present models of ionized gas to be used for the interpretation of active galactic nuclei and young star-forming galaxies. The community is encouraged to make use of this electronic database and to perform a critical comparison between the individual datasets