Improved angular momentum evolution model for solar-like stars

F Gallet,J Bouvier

doi:10.1051/0004-6361/201525660

Abstract

We developed angular momentum evolution models for 0.5 and 0.8 $M_{\odot}$ stars. The parametric models include a new wind braking law based on recent numerical simulations of magnetised stellar winds, specific dynamo and mass-loss rate prescriptions, as well as core/envelope decoupling. We compare model predictions to the distributions of rotational periods measured for low mass stars belonging to star forming regions and young open clusters. Furthermore, we explore the mass dependence of model parameters by comparing these new models to the solar-mass models we developed earlier. Rotational evolution models are computed for slow, median, and fast rotators at each stellar mass. The models reproduce reasonably well the rotational behaviour of low-mass stars between 1 Myr and 8-10 Gyr, including pre-main sequence to zero-age main sequence spin up, prompt zero-age main sequence spin down, and early-main sequence convergence of the surface rotation rates. Fast rotators are found to have systematically shorter disk lifetimes than moderate and slow rotators, thus enabling dramatic pre-main sequence spin up. They also have shorter core-envelope coupling timescales, i.e., more uniform internal rotation. As to the mass dependence, lower mass stars require significantly longer core-envelope coupling timescale than solar-type ones, which results in strong differential rotation developing in the stellar interior on the early main sequence. Lower mass stars also require a weaker braking torque to account for their longer spin down timescale on the early main sequence, while they ultimately converge towards lower rotational velocities than solar-type stars on the longer term due to their reduced moment of inertia. We also find evidence that the mass-dependence of the wind braking efficiency may be related to a change of the magnetic topology in lower mass stars.

Highlights

Observational constraints on the rotational evolution of lowmass stars have exponentially increased in the last decade, thanks to a number of dedicated long-term monitoring studies of nearby populations
The models developed in the previous section show that these trends can be well reproduced with a small number of assumptions: i) a magnetic star-disk interaction that is believed to prevent the stars from spinning up during the early pre-main sequence (PMS), which results in a nearly constant rotation rate during the disk accretion phase; ii) angular momentum losses due to magnetised stellar winds, a process whose magnitude depends on stellar mass, magnetic field, and rotation rate, and which dominates the rotational evolution of low-mass stars past the ZAMS; and iii) angular momentum redistribution in the stellar interior, which allows a large fraction of the initial angular momentum to be temporarily stored in the inner radiative core on the early main sequence (MS)
The rotational evolution of low-mass stars in the range from 0.5 to 1.1 M can be described, from birth to the end of the main sequence by parametric models that rely on a limited number of physical processes: star-disk interaction, wind braking, and core-envelope decoupling

Summary

Introduction

Observational constraints on the rotational evolution of lowmass stars have exponentially increased in the last decade, thanks to a number of dedicated long-term monitoring studies of nearby populations (see e.g. Irwin & Bouvier 2009; Hartman et al 2010; Agüeros et al 2011; Meibom et al 2011a; Irwin et al 2011; Affer et al 2012, 2013; Bouvier et al 2014; Gallet & Bouvier 2013; McQuillan et al 2014). Irwin & Bouvier 2009; Hartman et al 2010; Agüeros et al 2011; Meibom et al 2011a; Irwin et al 2011; Affer et al 2012, 2013; Bouvier et al 2014; Gallet & Bouvier 2013; McQuillan et al 2014) These new observational results provide extremely useful guidance for the modelling of angular momentum evolution of low-mass stars (M∗ < 1.2 M ) from 1 Myr to 10 Gyr (e.g. Irwin et al 2007; Bouvier 2008; Denissenkov et al 2010; Spada et al 2011; Reiners & Mohanty 2012; Gallet & Bouvier 2013) and offer a unique insight into the physical processes that dictate rotational evolution. The aim of the present study is Article published by EDP Sciences

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Discussion

Conclusion