The energy budget of stellar magnetic fields
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Spectropolarimetric observations have been used to map stellar magnetic fields, many of which display strong bands of azimuthal fields that are toroidal. A number of explanations have been proposed to explain how such fields might be generated though none are definitive. In this paper, we examine the toroidal fields of a sample of 55 stars with magnetic maps, with masses in the range 0.1–1.5 M⊙. We find that the energy contained in toroidal fields has a power-law dependence on the energy contained in poloidal fields. However the power index is not constant across our sample, with stars less and more massive than 0.5 M⊙ having power indices of 0.72 ± 0.08 and 1.25 ± 0.06, respectively. There is some evidence that these two power laws correspond to stars in the saturated and unsaturated regimes of the rotation-activity relation. Additionally, our sample shows that strong toroidal fields must be generated axisymmetrically. The latitudes at which these bands appear depend on the stellar rotation period with fast rotators displaying higher latitude bands than slow rotators. The results in this paper present new constraints for future dynamo studies.
See , W C V , Jardine , M M , Vidotto , A , Donati , J-F , Folsom , C P , Boro Saikia , S , Bouvier , J , Fares , R , Gregory , S , Hussain , G A J , Jeffers , S V , Marsden , S C , Morin , J , Moutou , C , do Nascimento , J D , Petit , P , Rosén , L & Waite , I A 2015 , ' The energy budget of stellar magnetic fields ' Monthly Notices of the Royal Astronomical Society , vol 453 , no. 4 , pp. 4301-4310 . DOI: 10.1093/mnras/stv1925
Monthly Notices of the Royal Astronomical Society
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, © 2015 The Authors, Published by Oxford University Press on behalf of the Royal Astronomical Society. This work is made available online in accordance with the publisher’s policies. This is the final published version of the work, which was originally published at https://dx.doi.org/10.1093/mnras/stv1925
VS acknowledges the support of an Science & Technology Facilities Council (STFC) studentship. AAV acknowledges support from the Swiss National Science Foundation through an Ambizione Fellowship. SBS and SVJ acknowledge research funding by the Deutsche Forchungsgemeinschaft (DFG) under grant SFB, project A16. SGG acknowledges support from the STFC via an Ernest Rutherford Fellowship [ST/J003255/1]. This study was supported by the grant ANR 2011 Blanc SIMI5-6 020 01 ‘Toupies: Towards understanding the spin evolution of stars’ (http://ipag.osug.fr/Anr_Toupies/).