Rotational evolution of solar-type protostars during the star-disk interaction phase

F Gallet,L Amard,C Zanni

doi:10.1051/0004-6361/201935432

Abstract

Context. The early pre-main sequence phase during which solar-mass stars are still likely surrounded by an accretion disk represents a puzzling stage of their rotational evolution. While solar-mass stars are accreting and contracting, they do not seem to spin up substantially. Aims. It is usually assumed that the magnetospheric star-disk interaction tends to maintain the stellar rotation period constant (“disk-locking”), but this hypothesis has never been thoroughly verified. Our aim is to investigate the impact of the star-disk interaction mechanism on the stellar spin evolution during the accreting pre-main sequence phases. Methods. We devised a model for the torques acting on the stellar envelope based on studies of stellar winds, and we developed a new prescription for the star-disk coupling founded on numerical simulations of star-disk interaction and magnetospheric ejections. We then used this torque model to follow the long-term evolution of the stellar rotation. Results. Strong dipolar magnetic field components up to a few kG are required to extract enough angular momentum so as to keep the surface rotation rate of solar-type stars approximately constant for a few Myr. Furthermore an efficient enough spin-down torque can be provided by either one of the following: a stellar wind with a mass outflow rate corresponding to ≈10% of the accretion rate, or a lighter stellar wind combined with a disk that is truncated around the corotation radius entering a propeller regime. Conclusions. Magnetospheric ejections and accretion powered stellar winds play an important role in the spin evolution of solar-type stars. However, kG dipolar magnetic fields are neither uncommon or ubiquitous. Besides, it is unclear how massive stellar winds can be powered while numerical models of the propeller regime display a strong variability that has no observational confirmation. Better observational statistics and more realistic models could contribute to help lessen our calculations’ requirements.

Highlights

Classical T Tauri stars (CTTS) are magnetically active pre-main sequence stars surrounded by an accretion disk (Collier Cameron & Campbell 1993; Edwards et al 1993, 1994; Collier Cameron et al 1995; Hartmann et al 1998)
The early pre-main sequence phase during which solar-mass stars are still likely surrounded by an accretion disk represents a puzzling stage of their rotational evolution
We devised a model for the torques acting on the stellar envelope based on studies of stellar winds, and we developed a new prescription for the star-disk coupling founded on numerical simulations of star-disk interaction and magnetospheric ejections

Summary

Introduction

Classical T Tauri stars (CTTS) are magnetically active pre-main sequence stars surrounded by an accretion disk (Collier Cameron & Campbell 1993; Edwards et al 1993, 1994; Collier Cameron et al 1995; Hartmann et al 1998). Observations suggest (see Edwards et al 1993; Bouvier et al 1993; Rebull et al 2004; Irwin & Bouvier 2009) that while stars contract during the early pre-main sequence (PMS) phase (1–10 Myr) and accrete angular momentum from the disk, they do not seem to noticeably spin up for several Myr. Gallet & Bouvier (2013; 2015, and references therein) highlight the apparent steady evolution of the three percentiles (90th, median, and 25th) of the rotational period distributions of stars from 1 Myr to 10 Myr. The fact that the two extreme percentiles remain almost constant in time confirms that the evolution of the rotation is not random but strongly depends on the initial conditions, that is, whether the star is initially in a fast or slow rotating regime. Regardless of the physical origin of this rotational evolution, these observational constraints suggest that during this early-PMS phase, a large fraction of the angular momentum of the star needs to be removed

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Results

Conclusion