Abstract
ABSTRACT The time evolution of angular momentum and surface rotation of massive stars are strongly influenced by fossil magnetic fields via magnetic braking. We present a new module containing a simple, comprehensive implementation of such a field at the surface of a massive star within the Modules for Experiments in Stellar Astrophysics (mesa) software instrument. We test two limiting scenarios for magnetic braking: distributing the angular momentum loss throughout the star in the first case, and restricting the angular momentum loss to a surface reservoir in the second case. We perform a systematic investigation of the rotational evolution using a grid of OB star models with surface magnetic fields (M⋆ = 5–60 M⊙, Ω/Ωcrit = 0.2–1.0, Bp = 1–20 kG). We then employ a representative grid of B-type star models (M⋆ = 5, 10, 15 M⊙, Ω/Ωcrit = 0.2, 0.5, 0.8, Bp = 1, 3, 10, 30 kG) to compare to the results of a recent self-consistent analysis of the sample of known magnetic B-type stars. We infer that magnetic massive stars arrive at the zero-age main sequence (ZAMS) with a range of rotation rates, rather than with one common value. In particular, some stars are required to have close-to-critical rotation at the ZAMS. However, magnetic braking yields surface rotation rates converging to a common low value, making it difficult to infer the initial rotation rates of evolved, slowly rotating stars.
Highlights
Magnetic fields are routinely detected in stars across the entire Hertzsprung–Russell diagram (HRD), from early to late evolutionary phases (Donati & Landstreet 2009)
We described the implementation of magnetic braking applicable for hot, massive stars in the Modules for Experiments in Stellar Astrophysics (MESA) software instrument, and studied the rotational evolution of the models
We provide the scientific community with this additional MESA module that contains a realistic and simple prescription of surface fossil magnetic fields in stellar evolution codes
Summary
Magnetic fields are routinely detected in stars across the entire Hertzsprung–Russell diagram (HRD), from early to late evolutionary phases (Donati & Landstreet 2009). Surface magnetic fields have a complex interaction with stellar winds, confining the wind material along closed magnetic field lines (Babel & Montmerle 1997; ud-Doula & Owocki 2002; Owocki & ud-Doula 2004; Townsend & Owocki 2005; Bard & Townsend 2016) This interaction leads to two dynamical effects that have a considerable impact over evolutionary time-scales: mass-loss quenching, which reduces the effective mass-loss rate of the star, and magnetic braking, which reduces the angular momentum of the star. While this term is often used in different contexts, in the following we refer to magnetic braking to describe the rotational spin-down of hot, massive stars caused by a large-scale dipolar surface fossil field.
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