Angular Momentum Evolution of Stars in the Orion Nebula Cluster

Abstract

We present theoretical models of stellar angular momentum evolution from the Orion Nebula Cluster (ONC) to the Pleiades and the Hyades. We demonstrate that observations of the Pleiades and Hyades place tight constraints on the angular momentum loss rate from stellar winds. The observed periods, masses, and ages of ONC stars in the range 0.2-0.5 M☉ and the loss properties inferred from the Pleiades and Hyades stars are then used to test the initial conditions for stellar evolution models. We use these models to estimate the distribution of rotational velocities for the ONC stars at the age of the Pleiades (120 Myr). The modeled ONC and observed Pleiades distributions of rotation rates are not consistent if only stellar winds are included. In order to reconcile the observed loss of angular momentum between these two clusters, an extrinsic loss mechanism, such as protostar-accretion disk interaction, is required. Our model, which evolves the ONC stars with a mass-dependent saturation threshold, normalized such that ωcrit = 5.4 ω☉ at 0.5 M☉, and includes a distribution of disk lifetimes that is uniform over the range 0-6 Myr, is consistent with the Pleiades data. This model for disk-locking lifetimes is also consistent with disk lifetimes inferred from the percentage of stars with infrared excesses observed in young clusters. Different models, using a variety of initial period distributions and different maximum disk lifetimes, are also compared with the Pleiades. For disk-locking models that use the ONC period distribution and a uniform distribution of disk lifetimes over the range 0 to τmax, the acceptable range of the maximum lifetime is 3.5 Myr < τmax < 8.5 Myr. We compare these results with birth line models that use different starting period distributions.