1998A&A...337..403B

We present evolutionary models for low mass stars from 0.075 to 1M_☉ for solar-type metallicities [M/H]=0 and -0.5. The calculations include the most recent interior physics and the latest generation of non-grey atmosphere models. We provide mass-age-color-magnitude relationships for both metallicities. The mass-M_V and mass-M_K relations are in excellent agreement with the empirical relations derived observationally. The theoretical color-magnitude diagrams are compared with the sequences of globular clusters (47 Tucanae) and open clusters (NGC 2420 and NGC 2477) observed with the Hubble Space Telescope. Comparison is also made with field star sequences in M_V-(V-I), M_K-(I-K) and M_K-(J-K) diagrams. These comparisons show that the most recent improvements performed in low-mass star atmosphere models yield now reliable stellar models in the near-infrared. These models can be used for metallicity, mass, temperature and luminosity calibrations. Uncertainties still remain, however, in the optical spectral region below T_eff∼3700K, where predicted (V-I) colors are too blue by 0.5mag for a given magnitude. The possible origins for such a discrepancy, most likely a missing source of opacity in the optical and the onset of grain formation are examined in detail.