TY - JOUR
T1 - Do Non-dipolar Magnetic Fields Contribute to Spin-down Torques?
AU - See, Victor
AU - Matt, Sean P.
AU - Finley, Adam J.
AU - Folsom, Colin P.
AU - Saikia, Sudeshna Boro
AU - Donati, Jean Francois
AU - Fares, Rim
AU - Hébrard, Élodie M.
AU - Jardine, Moira M.
AU - Jeffers, Sandra V.
AU - Marsden, Stephen C.
AU - Mengel, Matthew W.
AU - Morin, Julien
AU - Petit, Pascal
AU - Vidotto, Aline A.
AU - Waite, Ian A.
N1 - Publisher Copyright:
© 2019. The American Astronomical Society. All rights reserved.
PY - 2019/12/1
Y1 - 2019/12/1
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=85075304150&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85075304150&partnerID=8YFLogxK
U2 - 10.3847/1538-4357/ab46b2
DO - 10.3847/1538-4357/ab46b2
M3 - Article
AN - SCOPUS:85075304150
SN - 0004-637X
VL - 886
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 2
M1 - 120
ER -