Do non-dipolar magnetic fields contribute to spin-down torques?
See, V; Matt, SP; Finley, AJ; et al.Folsom, CP; Saikia, SB; Donati, J-F; Fares, R; Hébrard, ÉM; Jardine, MM; Jeffers, SV; Marsden, SC; Mengel, MW; Morin, J; Petit, P; Vidotto, AA; Waite, IA
Date: 27 November 2019
Journal
Astrophysical Journal
Publisher
American Astronomical Society / IOP Publishing
Publisher DOI
Abstract
Main sequence low-mass stars are known to spin-down as a consequence of their magnetised stellar winds. However, estimating the precise rate of this spin-down is an open problem. The mass-loss rate, angular momentum-loss rate and the magnetic field properties of low-mass stars are fundamentally linked making this a challenging task. ...
Main sequence low-mass stars are known to spin-down as a consequence of their magnetised stellar winds. However, estimating the precise rate of this spin-down is an open problem. The mass-loss rate, angular momentum-loss rate and the magnetic field properties of low-mass stars are fundamentally linked making this a challenging task. Of particular interest is the stellar magnetic field geometry. In this work, we consider whether non-dipolar field modes contribute significantly to the spin-down of low-mass stars. We do this using a sample of stars that have all been previously mapped with Zeeman-Doppler imaging. For a given star, as long as its mass-loss rate is below some critical mass-loss rate, only the dipolar fields contribute to its spin-down torque. However, if it has a larger mass-loss rate, higher order modes need to be considered. For each star, we calculate this critical mass-loss rate, which is a simple function of the field geometry. Additionally, we use two methods of estimating mass-loss rates for our sample of stars. In the majority of cases, we find that the estimated mass-loss rates do not exceed the critical mass-loss rate and hence, the dipolar magnetic field alone is sufficient to determine the spin-down torque. However, we find some evidence that, at large Rossby numbers, non-dipolar modes may start to contribute.
Physics and Astronomy
Faculty of Environment, Science and Economy
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