Investigating the rotational evolution of young, low-mass stars using Monte Carlo simulations

Abstract

Context. Young stars rotate well below break-up velocity, which is thought to result from the magnetic coupling with their accretion disk. Aims. We investigate the rotational evolution of young stars under the disk-locking hypothesis through Monte Carlo simulations. Methods. Our simulations included 280 000 stars, each of which was initially assigned a mass, a rotational period, and a mass accretion rate. The mass accretion rate depends on both mass and time, following power-law indices of 1.4 and −1.5, respectively. A mass-dependent accretion threshold was defined below which a star was considered as diskless, which resulted in a distribution of disk lifetimes that matches observations. Stars were evolved at constant angular spin rate while accreting and at constant angular momentum when they became diskless. Results. Starting with a bimodal distribution of periods for disk and diskless stars, we recovered the bimodal period distribution seen in several young clusters. The short-period peak mostly consists of diskless stars, and the long-period peak is mainly populated by accreting stars. Both distributions, however, present a long tail toward long periods, and a population of slowly rotating diskless stars is observed at all ages. We reproduced the observed correlations between disk fraction and spin rate, as well as between IR excess and rotational period. The period-mass relation we derived from the simulations only shows the same global trend as observed in young clusters when we released the disk-locking assumption for the lowest mass stars. Finally, we find that the time evolution of median specific angular momentum follows a power-law index of −0.65 for accreting stars, as expected from disk locking, and of −0.53 for diskless stars, a shallower slope that results from a wide distribution of disk lifetimes. At the end of the accretion phase, our simulations reproduce the angular momentum distribution of the low-mass members of the 13 Myr h Per cluster. Conclusions. Using observationally documented distributions of disk lifetimes, mass accretion rates, and initial rotation periods, and evolving an initial population from 1 to 12 Myr, we reproduced the main characteristics of pre-main sequence angular momentum evolution, which supports the disk-locking hypothesis.

Stellar models simulating the disk-locking mechanism and the evolutionary history of the Orion Nebula cluster and NGC 2264

“…4.3) is a promising scenario to explain the period distribution of NGC 2264. This interpretation is supported by Monte Carlo simulations results by Vasconcelos & Bouvier (2015), who used similar disk-locking hypotheses and reproduced the disk fraction as a function of period of ONC and NGC 2264 stars and their accretion-rotation and mass-rotation connections.…”

Near-infrared time-series photometry in the field of Cygnus OB2 association

“…Article number, page 36 of 51 corroborate the disk-locking results for periods up to 12.6 days, even though the increase of the disk fraction with period occurs in a less accentuate way when compared with Vasconcelos & Bouvier (2015) results. For periods longer than that, i.e.…”

“…Their results for 5.1 Myr, and 10.1 Myr are shown in the bottom plot in Figure 20, together with the results for our sample. In both plots in Figure 20 vertical bars follow standard errors of a Poisson counting, as the error bars used by Vasconcelos & Bouvier (2015). Results from Guarcello et al (2016, hereafter GDW16) investigation on the disk-survival in CygOB2 suggest that the environmental feedback on disk evolution for CygOB2 members may be responsible for a decrease of about 20% on the disk fraction.…”

“…In this case, the disk lifetime distribution in CygOB2 would be very different from the ones used by Vasconcelos & Bouvier (2015), since they assume in their simulations that most stars are born with disks, and suffer from a smooth decrease on disk-fraction with time. Consequently, a comparison with their results would not be possible.…”

CSI 2264: Investigating rotation and its connection with disk accretion in the young open cluster NGC 2264

“…The comparison of the final period distribution simulated with observational data for clusters of similar age therefore enables investigating what processes regulate the spin evolution of young stars, especially in the earliest stages. Vasconcelos & Bouvier (2015) presented Monte Carlo simulations of the rotational evolution of a population of 280,000 young stars with mass between 0.3 and 1 M d , from an age of 1 Myr to an age of 12 Myr. Their models assume that stars evolve at a constant angular velocity when they are coupled to an active accretion disk, and at a constant angular momentum when the disk is dissipated.…”