Improving PARSEC models for very low mass stars

Many stellar models present difficulties in reproducing basic observational relations of very low mass stars (VLMS), including the mass-radius relation and the optical colour-magnitudes of cool dwarfs. Here, we improve PARSEC (PAdova-TRieste Stellar Evolution Code) models on these points. We implement the T-tau relations from PHOENIX BT-Settl model atmospheres as the outer boundary conditions in the PARSEC code, finding that this change alone reduces the discrepancy in the mass-radius relation from 8 to 5 per cent. We compare the models with multiband photometry of clusters Praesepe and M67, showing that the use of T-tau relations clearly improves the description of the optical colours and magnitudes. But anyway, using both Kurucz and PHOENIX model spectra, model colours are still systematically fainter and bluer than the observations. We then apply a shift to the above T-tau relations, increasing from 0 at T-eff = 4730 K to similar to 14 per cent at T-eff = 3160 K, to reproduce the observed mass-radius relation of dwarf stars. Taking this experiment as a calibration of the T-tau relations, we can reproduce the optical and near-infrared colour-magnitude diagrams of low-mass stars in the old metal-poor globular clusters NGC 6397 and 47 Tuc, and in the intermediate-age and young solar-metallicity open clusters M67 and Praesepe. Thus, we extend PARSEC models using this calibration, providing VLMS models more suitable for the lower main-sequence stars over a wide range of metallicities and wavelengths. Both sets of models are available on PARSEC webpage.