The coronal structure of Speedy Mic - II: Prominence masses and off-disc emission

Observations of stellar prominences on young rapidly rotating stars provide unique probes of their magnetic fields out to many stellar radii. We compare two independently obtained datasets of the K3 dwarf Speedy Mic (BOMic, HD 197890) using the Anglo-Australian Telescope (AAT) and the ESO Very Large Telescope (VLT). Taken more than a fortnight apart they provide the first insight into the evolution of the prominence system on such a young rapidly rotating star. The largest prominences observed transiting the stellar disc are found at very similar rotational phases between the epochs. This suggests that the magnetic structures supporting the prominences retain their identity on a two to three week timescale. By taking advantage of the high signal-to-noise and large wavelength range of the VLT observations we identify prominences as transient absorption features in all lines of the Hydrogen Balmer series down to H10. We use the ratios of the prominence EWs in these lines to determine their column densities in the first excited state of hydrogen.We determine the optical depths, finding prominences to be rather optically thick (τ 20) in the H line. The total hydrogen column density and thus the prominence masses are determined via observations of the CaII H&K lines. We find typical masses for four of the largest prominences to be in the range 0.5 − 2.3 x1014 kg, slightly larger than giant solar prominence masses. Rotationally modulated emission is seen outside of the H line. These loops of emission are shown to be caused by prominences seen off the stellar disc. We find that all of the large emission loops can be associated with prominences we see transiting the stellar disc. This combined with the fact that many prominences appear to eclipse the off-disc emission of others, strongly suggests that the prominence system is highly flattened and likely confined to low stellar latitudes.