Do non-dipolar magnetic fields contribute to spin-down torques?

See, Victor and Matt, Sean P and Finley, Adam J and Folsom, Colin P and Boro Saikia, Sudeshna and Donati, Jean-Francois and Fares, Rim and Hebrard, Elodie M and Jardine, Moira M and Jeffers, Sandra V and Marsden, Stephen C and Mengel, Matthew W and Morin, Julien and Petit, Pascal and Vidotto, Aline A and Waite, Ian W (2019) Do non-dipolar magnetic fields contribute to spin-down torques? The Astrophysical Journal, 886 (120). ISSN 1538-4357

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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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Item Type: Article (Commonwealth Reporting Category C)
Refereed: Yes
Item Status: Live Archive
Faculty/School / Institute/Centre: Current - Institute for Advanced Engineering and Space Sciences - Centre for Astrophysics (1 Aug 2018 -)
Faculty/School / Institute/Centre: Current - Faculty of Health, Engineering and Sciences - School of Sciences (6 Sep 2019 -)
Date Deposited: 29 Jan 2020 05:25
Last Modified: 30 Jan 2020 05:48
Uncontrolled Keywords: magnetohydrodynamics (MHD) – stars: evolution – stars: low-mass – stars: magnetic field – stars: rotation – stars: winds, outflows
Fields of Research (2008): 02 Physical Sciences > 0201 Astronomical and Space Sciences > 020110 Stellar Astronomy and Planetary Systems
Socio-Economic Objectives (2008): E Expanding Knowledge > 97 Expanding Knowledge > 970102 Expanding Knowledge in the Physical Sciences
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