The contribution of starspots to coronal structure

TL;DR

This paper investigates how starspots influence the extrapolated coronal magnetic field structures of stars, revealing that spots can significantly alter magnetic flux estimates and related stellar activity indicators.

Contribution

It models the magnetic field within starspots to assess their impact on coronal structure, a novel approach that accounts for previously censored magnetic regions.

Findings

Starspots can nearly double the open magnetic flux in certain stars.

Spot coverage and distribution significantly affect coronal magnetic field estimates.

Including spot fields alters predictions of stellar wind and X-ray emission.

Abstract

Significant progress has been made recently in our understanding of the structure of stellar magnetic fields, thanks to advances in detection methods such as Zeeman-Doppler Imaging. The extrapolation of this surface magnetic field into the corona has provided 3D models of the coronal magnetic field and plasma. This method is sensitive mainly to the magnetic field in the bright regions of the stellar surface. The dark (spotted) regions are censored because the Zeeman signature there is suppressed. By modelling the magnetic field that might have been contained in these spots, we have studied the effect that this loss of information might have on our understanding of the coronal structure. As examples, we have chosen two stars (V374 peg and AB Dor) that have very different magnetograms and patterns of spot coverage. We find that the effect of the spot field depends not only on the relative amount of flux in the spots, but also its distribution across the stellar surface. For a star such as AB Dor with a high spot coverage and a large polar spot, at its greatest effect the spot field may almost double the fraction of the flux that is open (hence decreasing the spindown time) while at the same time increasing the X-ray emission measure by two orders of magnitude and significantly affecting the X-ray rotational modulation.