Rotational evolution of slowrotator sequence stars
Abstract
Context. The observed relationship between mass, age and rotation in open clusters shows the progressive development of a slowrotator sequence among stars possessing a radiative interior and a convective envelope during their premain sequence and mainsequence evolution. After 0.6 Gyr, most cluster members of this type have settled on this sequence.
Aims: The observed clustering on this sequence suggests that it corresponds to some equilibrium or asymptotic condition that still lacks a complete theoretical interpretation, and which is crucial to our understanding of the stellar angular momentum evolution.
Methods: We couple a rotational evolution model, which takes internal differential rotation into account, with classical and new proposals for the wind braking law, and fit models to the data using a Monte Carlo Markov chain (MCMC) method tailored to the problem at hand. We explore to what extent these models are able to reproduce the mass and time dependence of the stellar rotational evolution on the slowrotator sequence.
Results: The description of the evolution of the slowrotator sequence requires taking the transfer of angular momentum from the radiative core to the convective envelope into account. We find that, in the mass range 0.851.10 M_{⊙}, the coreenvelope coupling timescale for stars in the slowrotator sequence scales as M^{7.28}. Quasisolid body rotation is achieved only after 12 Gyr, depending on stellar mass, which implies that observing small deviations from the Skumanich law (P ∝ √{t}) would require period data of older open clusters than is available to date. The observed evolution in the 0.12.5 Gyr age range and in the 0.851.10 M_{⊙} mass range is best reproduced by assuming an empirical mass dependence of the wind angular momentum loss proportional to the convective turnover timescale and to the stellar moment of inertia. Period isochrones based on our MCMC fit provide a tool for inferring stellar ages of solarlike mainsequence stars from their mass and rotation period that is largely independent of the wind braking model adopted. These effectively represent gyrochronology relationships that take the physics of the twozone model for the stellar angular momentum evolution into account.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 December 2015
 DOI:
 10.1051/00046361/201526770
 arXiv:
 arXiv:1506.05298
 Bibcode:
 2015A&A...584A..30L
 Keywords:

 stars: rotation;
 stars: evolution;
 stars: latetype;
 open clusters and associations: general;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 16 pages, 8 figures, 4 tables, accepted by A&