Could a change in magnetic field geometry cause the break in the wind-activity relation?

A. A. Vidotto; J. -F. Donati; M. Jardine; V. See; P. Petit; I. Boisse; S. Boro Saikia; E. Hebrard; S. V. Jeffers; S. C. Marsden; J. Morin

doi:10.1093/mnrasl/slv147

paper

Could a change in magnetic field geometry cause the break in the wind-activity relation?

arXiv:1509.08751 · doi:10.1093/mnrasl/slv147

Abstract

Wood et al suggested that mass-loss rate is a function of X-ray flux ( $M ˙ \propto F_{x}^{1.34}$ ) for dwarf stars with $F_{x} ≲ F_{x, 6} \equiv 1 0^{6}$ erg cm $^{- 2}$ s $^{- 1}$ . However, more active stars do not obey this relation. These authors suggested that the break at $F_{x, 6}$ could be caused by significant changes in magnetic field topology that would inhibit stellar wind generation. Here, we investigate this hypothesis by analysing the stars in Wood et al's sample that had their surface magnetic fields reconstructed through Zeeman-Doppler Imaging (ZDI). Although the solar-like outliers in the $M ˙$ -- $F_{x}$ relation have higher fractional toroidal magnetic energy, we do not find evidence of a sharp transition in magnetic topology at $F_{x, 6}$ . To confirm this, further wind measurements and ZDI observations at both sides of the break are required. As active stars can jump between states with highly toroidal to highly poloidal fields, we expect significant scatter in magnetic field topology to exist for stars with $F_{x} ≳ F_{x, 6}$ . This strengthens the importance of multi-epoch ZDI observations. Finally, we show that there is a correlation between $F_{x}$ and magnetic energy, which implies that $M ˙$ -- magnetic energy relation has the same qualitative behaviour as the original $M ˙$ -- $F_{x}$ relation. No break is seen in any of the $F_{x}$ -- magnetic energy

7 pages, 3 figures, 4 tables, accepted to MNRAS Letters

Could a change in magnetic field geometry cause the break in the wind-activity relation?

Abstract

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