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

Victor See, Sean P. Matt, Adam J. Finley, Colin P. Folsom, Sudeshna Boro Saikia, Jean Francois Donati, Rim Fares, Élodie M. Hébrard, Moira M. Jardine, Sandra V. Jeffers, Stephen C. Marsden, Matthew W. Mengel, Julien Morin, Pascal Petit, Aline A. Vidotto, Ian A. Waite

Research output: Contribution to journalArticlepeer-review

47 Citations (Scopus)


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.

Original languageEnglish
Article number120
JournalAstrophysical Journal
Issue number2
Publication statusPublished - Dec 1 2019

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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