Scaled solar tracks and isochrones in a large region of the Z-Y plane - I. From the ZAMS to the TP-AGB end for 0.15-2.5 Msun stars

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 Msun 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_sun) 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.