Improved angular momentum evolution model for solar-like stars

F Gallet,J Bouvier

doi:10.1051/0004-6361/201321302

Abstract

We present new models for the rotational evolution of solar-like stars between 1 Myr and 10 Gyr with the aim to reproduce the distributions of rotational periods observed for star forming regions and young open clusters within this age range. The models include a new wind braking law based on recent numerical simulations of magnetized stellar winds and specific dynamo and mass-loss prescriptions are adopted to tie angular momentum loss to angular velocity. The model additionally assume constant angular velocity during the disk accretion phase and allow for decoupling between the radiative core and the convective envelope as soon as the former develops. We have developed rotational evolution models for slow, median and fast rotators with initial periods of 10, 7, and 1.4d, respectively. The models reproduce reasonably well the rotational behaviour of solar-type stars between 1 Myr and 4.5 Gyr, including PMS to ZAMS spin up, prompt ZAMS spin down, and the early-MS convergence of surface rotation rates. We find the model parameters accounting for the slow and median rotators are very similar to each other, with a disk lifetime of 5 Myr and a core-envelope coupling timescale of 28-30 Myr. In contrast, fast rotators have both shorter disk lifetime (2.5 Myr) and core-envelope coupling timescale (12 Myr). We emphasize that these results are highly dependent on the adopted braking law. We also report a tentative correlation between initial rotational period and disk lifetime, which suggests that protostellar spin-down by massive disks in the embedded phase is at the origin of the initial dispersion of rotation rates in young stars. We conclude that this class of semi-empirical models successfully grasp the main trends of the rotational behaviour of solar-type stars as they evolve and make specific predictions that may serve as a guide for further development.

Highlights

IntroductionA wealth of new observational constraints have been gained from the derivation of complete rotational distributions for thousands of low mass stars in young open clusters and in the field, covering an age range from 1 Myr to about 10 Gyr (see, e.g., Irwin & Bouvier 2009; Hartman et al 2010; Agüeros et al 2011; Meibom et al 2011; Irwin et al 2011; Affer et al 2012, 2013)
The origin and evolution of stellar angular momentum still remains a mystery
We report a tentative correlation between the initial rotational period and disk lifetime, which suggests that protostellar spin down by massive disks in the embedded phase is at the origin of the initial dispersion of rotation rates in young stars

Summary

Introduction

A wealth of new observational constraints have been gained from the derivation of complete rotational distributions for thousands of low mass stars in young open clusters and in the field, covering an age range from 1 Myr to about 10 Gyr (see, e.g., Irwin & Bouvier 2009; Hartman et al 2010; Agüeros et al 2011; Meibom et al 2011; Irwin et al 2011; Affer et al 2012, 2013). In order to satisfy observational constraints, most of these models have to incorporate three major physical processes: stardisk interaction during the PMS, angular momentum loss to stellar winds, and redistribution of angular momentum in the stellar interior Each of these processes appears to have a fundamental role in dictating the evolution of surface rotation of solar-type stars from birth to the end of the main sequence and beyond

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Conclusion