Accretion-powered Stellar Winds. II. Numerical Solutions for Stellar Wind Torques

By S. Matt, R. Pudritz. 2008

148 citations recorded by the source corpus.

In order to explain the slow rotation observed in a large fraction of accreting pre-main-sequence stars (CTTSs), we explore the role of stellar winds in torquing down the stars. For this mechanism to be effective, the stellar winds need to have relatively high outflow rates, and thus would likely be powered by the accretion process itself. Here, we use numerical magnetohydrodynamical simulations to compute detailed two-dimensional (axisymmetric) stellar wind solutions, in order to determine the spin-down torque on the star. We discuss wind driving mechanisms and then adopt a Parker-like (thermal pressure driven) wind, modified by rotation, magnetic fields, and enhanced mass-loss rate (relative to the Sun). We explore a range of parameters relevant for CTTSs, including variations in the stellar mass, radius, spin rate, surface magnetic field strength, mass-loss rate, and wind acceleration rate. We also consider both dipole and quadrupole magnetic field geometries. Our simulations indicate that the stellar wind torque is of sufficient magnitude to be important for spinning down a “typical” CTTS, for a mass-loss rate of ~10−9 M☉ yr−1. The winds are wide-angle, self-collimated flows, as expected of magnetic rotator winds with moderately fast rotation. The cases with quadrupolar field produce a much weaker torque than for a dipole with the same surface field strength, demonstrating that magnetic geometry plays a fundamental role in determining the torque. Cases with varying wind acceleration rate show much smaller variations in the torque, suggesting that the details of the wind driving are less important. We use our computed results to fit a semianalytic formula for the effective Alfvén radius in the wind, as well as the torque. This allows for considerable predictive power, and is an improvement over existing approximations.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/b7f7933176b75a78e7aba3dd9e65d6c681b0fb4c

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Citations in the corpus

MAGNETIC CYCLES IN A CONVECTIVE DYNAMO SIMULATION OF A YOUNG SOLAR-TYPE STAR (2011, background): “…of a star off quickly with radius, the dipole mode dominates the magnetic energy distribution at large distances from the stellar surface, and so may contribute most to the magnetic “lever arm” that determines how rapidly a star’s rotation is braked (e.g., Weber & Davis 1967; Matt & Pudritz 2008).”
PERSISTENT MAGNETIC WREATHS IN A RAPIDLY ROTATING SUN (2010, background): “Magnetic fields can likewise feed back upon stellar rotation by modifying the rate at which angular momentum is lost through a stellar wind (e.g., Weber & Davis 1967; Matt & Pudritz 2008).”
Improved angular momentum evolution model for solar-like stars (2013, methodology): “This equation is a modified version of the Matt & Pudritz (2008a) prescription, rA = K3 ( B2pR 2 ∗ Ṁwindvesc )m R∗, (5) where K3 = 2.11 and m = 0.223 and are derived from numerical simulations.”
MHD simulations of accretion onto a dipolar magnetosphere - II. Magnetospheric ejections and stellar spin-down (2012, background): “(17) in Matt & Pudritz (2008b).”
THE MASS-DEPENDENCE OF ANGULAR MOMENTUM EVOLUTION IN SUN-LIKE STARS (2014, background): “The exponent factor m is determined primarily by the magnetic field geometry and wind acceleration profile (Réville et al. 2014) and likely falls in the range m =0.20–0.25 (Washimi & Shibata 1993; Matt & Pudritz 2008; Ud-Doula et al. 2009; Pinto et al. 2011; Matt et al. 2012).”
Improved angular momentum evolution model for solar-like stars II. Exploring the mass dependence (2015, unknown): “This behaviour is believed to result from the magnetic interaction between the young stellar object and its accretion disk, even though the actual physical process is not totally elucidated yet (see Matt & Pudritz 2005a, 2008a,b; Zanni & Ferreira 2011, 2013; Bouvier et al. 2014).”