1 Introduction

Models of evolutionary population synthesis require the computation of large data-bases of stellar evolutionary tracks, being as far as possible extended in their interval of initial masses and metallicities, covering the main evolutionary phases, and adopting a homogeneous and updated input physics.

One of the most popular of these sets is that from the Padova group (Bressan et al. 1993; Fagotto et al. 1994a,b; Bertelli et al. 1994, and references therein). The complete data-base covers a very large range of stellar masses (typically from 0.6 to 120 $M_{\odot }$) and metallicities (from Z=0.0004 to 0.05). The input physics is homogeneous for all stellar tracks in the grids. Main characteristics of these models are the adoption of OPAL opacities, a constant helium-to-metal enrichment ratio $\Delta Y/\Delta Z$, moderate convective overshooting from convective cores, and mass loss from massive stars. This data-base was extended to very low metallicities (i.e. Z=0.0001) by Girardi et al. (1996a).

Since 1994, many other evolutionary tracks have been computed by us, following some major updatings in the input physics. Some of the new tracks were intended only for testing the effects of these modifications, such as e.g. those discussed in Girardi et al. (1996b). The main novelties with respect to the Bertelli et al. (1994) tracks are, essentially, the adoption of an improved equation of state (Straniero 1988; Girardi et al. 1996a; Mihalas et al. 1990 and references therein), and the new low-temperature opacities from Alexander & Ferguson (1994). Eventually, complete sets of evolutionary tracks have been computed, thus generating a data-base which is comparable to the previous one in terms of the large coverage of mass and metallicity ranges.

This paper aims to present this new data-base of stellar evolutionary tracks and isochrones, and make it available to the users. The tracks presented here are limited to the interval of low- and intermediate mass stars (i.e. from 0.15 to 7 $M_{\odot }$), and to 6 values of metallicity, from Z=0.0004 to Z=0.03. Further extensions of this data-base will be provided in subsequent papers.

The new evolutionary tracks have already been used in several studies in the last years. They are the starting point for the detailed thermally pulsing AGB tracks of Marigo (1998a,b) and Marigo et al. (1998, 1999). The stellar chemical yields have been computed by Marigo (1998a) and Marigo et al. (in preparation). An extended sample of the low-mass ZAMS models has been used by Høg et al. (1998) and Pagel & Portinari (1998) in order to access the helium-to-metal enrichment ratio, $\Delta Y/\Delta Z$, for the local stars sampled by the astrometric satellite HIPPARCOS. Girardi (1996) and Girardi & Bertelli (1998) discussed the behaviour of the B-V and V-Kintegrated colours of single-burst stellar populations derived from the present tracks. Pasquini et al. (in preparation) used the evolution of the momentum of inertia of these models to interpretate observations of disc stars. Girardi et al. (1998) and Girardi (1999) generated synthetic colour-magnitude diagrams (CMD), finding substructures on the clump of red giants of different galaxy models. Finally, the present isochrones were used by Carraro et al. (1999a,b) to derive the ages of the oldest known open clusters.

The plane of this paper is as follows: Sect. 2 presents the input physics of the models; Sect. 3 introduces the stellar tracks and discuss their main characteristics; Sect. 4 describes the mass-loss on the RGB and synthetic TP-AGB evolution; Sect. 5 describes the derived isochrones. Finally Sect. 6 comments on the compatibility of present models with the Bertelli et al. (1994) data-base.