Stellar models simulating the disk-locking mechanism and the evolutionary history of the Orion Nebula cluster and NGC2264

Abstract

Rotational evolution in young stars is described by pMS evolutionary tracks including rotation, conservation of angular momentum (AM), and simulations of disk-locking (DL). By assuming that DL is the regulation mechanism for the stellar angular velocity during the early stages of pMS, we use our models and observational data to constrain disk lifetimes (Tdisk) of a sample of low-mass stars in the ONC and NGC2264. The period distributions of the ONC and NGC2264 are bimodal and depend on the stellar mass. To follow the rotational evolution of these two clusters' stars, we generated some sets of evolutionary tracks. We assumed that the evolution of fast rotators can be modeled by considering conservation of AM during all stages and of moderate rotators by considering conservation of angular velocity during the first stages of evolution. With these models we estimate a mass and an age for all stars. For the ONC, we assume that the secondary peak in the period distribution is due to high-mass objects locked in their disks, with a locking period (Plock) of ~8 days. For NGC2264 we make two hypotheses: (1) the stars in the secondary peak are locked with Plock=5 days, and (2) NGC2264 is in a later stage in the rotational evolution (this implies in a DL scenario with Plock=8 days, a Tdisk of 1 Myr and, after that, constant AM evolution). We simulated the period distribution of NGC2264 when its mean age was 1 Myr. Dichotomy and bimodality appear in the simulated distribution, presenting one peak at 2 days and another one at 5-7 days, indicating that the assumption of Plock=8 days is plausible. Our hypotheses are compared with observational disk diagnoses available in the literature. DL models with Plock=8 days and 0.2 Myr<=Tdisk<=3 Myr are consistent with observed periods of moderate rotators of the ONC. For NGC2264, hyphotesis 2 is the more promising explanation for its period distribution.