The coronal structure of Speedy Mic – II. Prominence masses and off-disc emission*

Abstract

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 data sets of the K3 dwarf Speedy Mic (BO Mic, HD 197890) using the Anglo‐Australian Telescope (AAT) and the European Southern Observatory (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 time‐scale. By taking advantage of the high signal‐to‐noise ratio 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 equivalent widths (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 Hα line. The total hydrogen column density and thus the prominence masses are determined via observations of the Ca II H&K lines. We find typical masses for four of the largest prominences to be in the range 0.5–2.3 × 1014 kg, slightly larger than giant solar prominence masses. Rotationally modulated emission is seen outside of the Hα 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.

Roche tomography of cataclysmic variables - IV. Star-spots and slingshot prominences on BV Cen

“…Cameron & Robinson 1989) and Speedy Mic (Dunstone et al 2006a) and are often observed as transient absorption features passing through the Doppler‐broadened Hα stellar line. Recently, however, an analysis of Very large Telescope data of Speedy Mic by Dunstone et al (2006b) has revealed rotationally modulated emission outside the stellar Hα line due to loops of emission seen off of the stellar disc, but which can also be associated with prominences seen to transit the stellar disc at other times. In addition, peculiar low‐velocity emission features seen in SS Cyg and IP Peg during outburst have also been interpreted as ‘slingshot prominences’ (Steeghs et al 1996).…”

Mass Loss in Pre-Main-Sequence Stars via Coronal Mass Ejections and Implications for Angular Momentum Loss

“…At later evolutionary stages, solar type stars are still seen to have extended, suspended material: the 50 Myr old (Luhman et al 2005) solar analog AB Dor was observed to have cool, corotating clouds at distances from ∼9-20 R * from the rotation axis, and were estimated to have masses 10 18 g (Collier Cameron & Robinson 1989). Dunstone et al (2006) observed prominences on Speedy Mic, a 30 Myr old K3 dwarf, and estimated masses to be ∼2×10 17 g. In stellar outflows, the Alfvén radii are typically comparable to or slightly larger than the size of the largest closed magnetic loops (e.g., Mestel & Spruit 1987). Thus, the measured sizes of postflare loops and prominences may represent an approximate lower limit on the Alfvén radii for these systems.…”

The highly spotted photosphere of the young rapid rotator Speedy Mic

“…Both studies found prominences at or above the corotation radius of the star. The Hα profile in the present data set is studied in a separate publication (Dunstone et al 2005), revealing a densely packed prominence system beyond the corotation radius.…”