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

doi:10.1093/mnras/staa297

paper

Measuring stellar magnetic helicity density

arXiv:2001.11749 · doi:10.1093/mnras/staa297

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 $_{⊙}$ 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 $R_{o} ≃ 0.1$ , 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: $∣ ⟨ h ⟩ ∣$ $\propto$ $⟨ B_{tor}_{2} ⟩^{0.86 \pm 0.04}$ . 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.

11 pages, 7 figures

Measuring stellar magnetic helicity density

Abstract

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