Non-gray rotating stellar models and the evolutionary history of the Orion Nebular Cluster

Abstract

Rotational evolution in the pre-main sequence (PMS) is described with new sets of PMS evolutionary tracks including rotation, non-gray boundary conditions (BCs) and either low (LCE) or high convection efficiency (HCE). Using observational data and our theoretical predictions, we aim at constraining 1) the differences obtained for the rotational evolution of stars within the ONC by means of these different sets of models; 2) the initial angular momentum of low mass stars, by means of their templates in the ONC. We discuss the reliability of current stellar models for the PMS. While the 2D radiation hydrodynamic simulations predict HCE in PMS, semi-empirical calibrations either seem to require that convection is less efficient in PMS than in the following MS phase or are still contradictory. We derive stellar masses and ages for the ONC by using both LCE and HCE. The resulting mass distribution for the bulk of the ONC population is in the range 0.2$-$0.3 {\msun} for our non-gray models and in the range 0.1$-$0.3{\msun} for models having gray BCs. In agreement with Herbst et al. (2002) we find that a large percentage ($\sim$70%) of low-mass stars (M\simlt 0.5{\msun} for LCE; M\simlt0.35{\msun} for HCE) in the ONC appears to be fast rotators (P$<$4days). Three possibilities are open: 1) $\sim$70% of the ONC low mass stars lose their disk at early evolutionary phases; 2)their locking period is shorter; 3) the period evolution is linked to a different morphology of the magnetic fields of the two groups of stars. We also estimate the range of initial angular momentum consistent with the observed periods. The comparisons made indicate that a second parameter is needed to describe convection in the PMS, possibly related to the structural effect of a dynamo magnetic field.