Rotational properties of young low mass stars through Monte Carlo simulations

Séminaire IPAG de Maria Jaqueline Vasconcelos (University of Brasília), jeudi 7 mai 2015 à 11h00, IPAG seminar room

Young, low mass stars present rotational periods that range from several hours to days. It is theoretically expected that stars that are surrounded by accretion disks rotate slower than diskless stars. However, observational results are controversial in this regard. In this work we present Monte Carlo simulations that evolve a population of stars from 1 Myr to 12.1 Myr. Each star has a mass from 0.3 M⊙ to 1.0 M⊙, a rotational period between 0.5 day to 18.5 days and an initial mass accretion rate M ̇ acc that depends on the mass but that is randomly chosen from a broad log-normal distribution. If M ̇ acc ≤ M ̇ acc,th, where M ̇ acc,th is the mass accretion rate threshold, the star is considered to be diskless. In this case, the stellar rotation period is evolved conserving angular momentum. Otherwise it is kept constant. Our results clearly show that most of the diskless stars rotate faster than disk stars but in order to reproduce the observational rotational properties seen in several young clusters the diskless stars should already rotate faster at 1 Myr. Although the median rotation period is different for disk and diskless stars, they don’t show a very different time evolution of their specific angular momentum j. While individually each diskless star conserves its specific angular momentum , ⟨j⟩ ∝ t−0.53 for the population of diskless stars. This is due to the wide distribution of disk lifetimes that is observed in our simulations. Our results show that with reasonable parameters and hypotheses we are able to reproduce most of the ro- tational properties of clusters younger than 20 Myr. Our results also support the disk locking hypothesis.