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Do Non-dipolar Magnetic Fields Contribute to Spin-down Torques?

Abstract : Main-sequence low-mass stars are known to spin down as a consequence of their magnetized stellar winds. However, estimating the precise rate of this spin-down is an open problem. The mass-loss rate, angular momentum loss rate, and 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; 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.
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Contributor : Jean-Francois Donati <>
Submitted on : Wednesday, November 25, 2020 - 5:48:33 PM
Last modification on : Tuesday, December 1, 2020 - 12:28:04 PM

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Victor See, Sean P. Matt, Adam J. Finley, Colin P. Folsom, Sudeshna Boro Saikia, et al.. Do Non-dipolar Magnetic Fields Contribute to Spin-down Torques?. The Astrophysical Journal, American Astronomical Society, 2019, ⟨10.3847/1538-4357/ab46b2⟩. ⟨hal-03024324⟩



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