2020MNRAS.491.4076J

The structure of a star's coronal magnetic field is a fundamental property that governs the high-energy emission from the hot coronal gas and the loss of mass and angular momentum in the stellar wind. It is, however, extremely difficult to measure. We report a new method to trace this structure in rapidly rotating young convective stars, using the cool gas trapped on coronal field lines as markers. This gas forms 'slingshot prominences' that appear as transient absorption features in H α. By using different methods of extrapolating this field from the surface measurements, we determine locations for prominence support and produce synthetic H α stacked spectra. The absorption features produced with a potential field extrapolation match well those observed, while the absorption features from a non-potential field do not. In systems where the rotation and magnetic axes are well aligned, up to 50 per cent of the prominence mass may transit the star and so produces a observable feature. This fraction may fall as low as ∼2 per cent in very highly inclined systems. Ejected prominences carry away mass and angular momentum at rates that vary by two orders of magnitude, but which may approach those carried by the stellar wind.