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arXiv · 2001.11749

Measuring stellar magnetic helicity density

K. Lund·M. Jardine·L. T. Lehmann·D. H. Mackay·V. See·A. A. Vidotto·J. -F. Donati·R. Fares·C. P. Folsom·S. V. Jeffers·S. C. Marsden·J. Morin·P. Petit
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Abstract

Helicity is a fundamental property of a magnetic field but to date it has only been possible to observe its evolution in one star - the Sun. In this paper we provide a simple technique for mapping the large-scale helicity density across the surface of any star using only observable quantities: the poloidal and toroidal magnetic field components (which can be determined from Zeeman-Doppler imaging) and the stellar radius. We use a sample of 51 stars across a mass range of 0.1-1.34 M$_\odot$ to show how the helicity density relates to stellar mass, Rossby number, magnetic energy and age. We find that the large-scale helicity density increases with decreasing Rossby number $R_o$, peaking at $R_o \simeq 0.1$, with a saturation or decrease below that. For both fully- and partially-convective stars we find that the mean absolute helicity density scales with the mean squared toroidal magnetic flux density according to the power law: $|\langle{h\,}\rangle|$ $\propto$ $\langle{\rm{B_{tor}}^2_{}\,\rangle}^{0.86\,\pm\,0.04}$. The scatter in this relation is consistent with the variation across a solar cycle, which we compute using simulations and observations across solar cycles 23 and 24 respectively. We find a significant decrease in helicity density with age.

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K. Lund, M. Jardine, L. T. Lehmann, D. H. Mackay, V. See, A. A. Vidotto, J. -F. Donati, R. Fares, C. P. Folsom, S. V. Jeffers, S. C. Marsden, J. Morin, P. Petit. 2020-01-31. Measuring stellar magnetic helicity density. https://doi.org/10.1093/mnras/staa297

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