The energy budget of stellar magnetic fields

TL;DR

This study analyzes the energy relationship between toroidal and poloidal magnetic fields in 55 stars, revealing mass-dependent power laws and implications for stellar dynamo theories.

Contribution

It provides the first detailed analysis of the energy dependence of stellar magnetic fields across a range of stellar masses, offering new constraints for dynamo models.

Findings

Toroidal field energy scales with poloidal energy via a mass-dependent power law.

Stars below and above 0.5 solar masses exhibit different power law indices.

Strong toroidal fields are generated axisymmetrically and relate to stellar rotation periods.

Abstract

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$\,{\rm M}_\odot$. 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$\,{\rm M}_\odot$ having power indices of 0.72$\pm$0.08 and 1.25$\pm$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.