The coronal structure of Speedy Mic - I. A densely packed prominence system beyond corotation

Abstract

We present new observations of the prominence system on the K3 dwarf Speedy Mic (BO Mic, HD 197890). Using an improved technique to track the absorption features in Hα we find a very active prominence system with approximately ten prominences on the observable hemisphere per rotation. From a total of 25 prominences we find an average axial distance of (2.85 ± 0.54)R which is twice the co-rotation radius above the stellar surface. We discuss the consequences of these observations on the nature of the supporting magnetic structures. Two consecutive nights, with complete phase coverage, combined with a further night after a three night gap allow us to study the evolution of the prominence system on two different timescales. Several of the prominences have counterparts at similar phases on consecutive nights. During this interval many prominences show evidence for evolution in their heights and phases of observation. Five nights (13 rotation cycles) later we recovermany prominences at approximately the same phases. Whilst individual prominences change axial distances or appear/re-appear from night-to-night the underlying prominence supporting structures appear to be stable over as many as 13 stellar rotations.