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.

We present a quantitative analysis of the star formation history (SFH) of the Local Group dSph galaxy Leo I, from the information in its Hubble Space Telescope [(V − I), I] color-magnitude diagram (CMD). It reaches the level of the oldest main-sequence turnoffs, and this allows us to retrieve the SFH in considerable detail. The method we use is based in comparing, via synthetic CMDs, the expected distribution of stars in the CMD for different evolutionary scenarios, with the observed distribution. We consider the SFH to be composed by the SFR(t), the chemical enrichment law Z(t), the initial mass function IMF, and a function β(f, q), controlling the fraction f and mass ratio distribution q of binary stars. We analyze a set of ≃ 50 combinations of four Z(t), three IMF and more than four β(f, q). For each of them, the best SFR(t) is searched for among ≃ 6 × 10 7 models. The comparison between the observed CMD and the model CMDs is done through χ 2 ν minimization of the differences in the number of stars in a set of regions of the CMD, chosen to sample stars of different ages or in specific stellar evolutionary phases. We empirically determine the range of χ 2 ν values that indicate acceptable models for our set of data using tests with models with known SFHs.Our solution for the SFH of Leo I defines a minimum of χ 2 ν in a well defined position of the parameter space, and the derived SFR(t) is robust, in the sense that its main characteristics are unchanged for different combinations of the remaining parameters. However, only a narrow range of assumptions for Z(t), IMF and β(f, q) result in a good agreement between the data and the models, namely: Z=0.0004, a Kroupa et al. (1993) IMF or slightly steeper, and a relatively large fraction of binary stars, with f = 0.3 − 0.6, q > 0.6 and approximately flat IMF for the secondaries, or particular combinations of these parameters that would produce a like fraction of similar mass binaries. Most star formation activity (70% to 80%) occurred between 7 and 1 Gyr ago. At 1 Gyr ago, it abruptly dropped to a negligible value, but seems to have been active until at least ≃ 300 million years ago. Our results don't unambiguously answer the question of whether Leo I began forming stars around 15 Gyr ago, but it appears that the amount of this star formation, if existing at all, would be small.

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