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Description
In recent years new observations of pre-main sequence stars (pre-MS) with Z<Z_{sun}_ have been made available. To take full advantage of the continuously growing amount of data of pre-MS stars in different environments, we need to develop updated pre-MS models for a wide range of metallicity to assign reliable ages and masses to the observed stars. We present updated evolutionary pre-MS models and isochrones for a fine grid of mass, age, metallicity, and helium values. We use a standard and well-tested stellar evolutionary code (i.e. FRANEC), that adopts outer boundary conditions from detailed and realistic atmosphere models. In this code, we incorporate additional improvements to the physical inputs related to the equation of state and the low temperature radiative opacities essential to computing low-mass stellar models. We make available via internet a large database of pre-MS tracks and isochrones for a wide range of chemical compositions (Z=0.0002-0.03), masses (M=0.2-7.0M_{sun}_), and ages (1-100Myr) for a solar-calibrated mixing length parameter alpha (i.e. 1.68). For each chemical composition, additional models were computed with two different mixing length values, namely alpha=1.2 and 1.9. Moreover, for Z>=0.008, we also provided models with two different initial deuterium abundances. The characteristics of the models have been discussed in detail and compared with other work in the literature. The main uncertainties affecting theoretical predictions have been critically discussed. Comparisons with selected data indicate that there is close agreement between theory and observation.
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