Theoretical isochrones compared to 2MASS observations: Open clusters at nearly solar metallicity

We study classical disk clusters at nearly solar metallicity with ages in the range 200 Myr – 6 Gyr using 2MASS photometry and theoretical isochrones. As a first step we employ J, H and K Johnson's Padova isochrones which, on a large scale, result in a good fit of the observed data. However, we find some significant deviations, especially in the red giant branch for intermediate age clusters and the low-mass main-sequence end in general. Subsequently, isochrones involving the 2MASS J, H and KS transmission curves were generated which account for differences of ≈0.02 of the above deviations, in the sense required by the 2MASS observations. However, these corrections are not enough to fully account for the observed differences. We suggest that the remaining deviations should be accounted for by model atmosphere limitations, particularly the lack of molecular opacity data in low-temperature giants. Future models will incorporate these updated data in order to produce isochrones suitable for the whole range of stellar masses.