Fast H variations on a rapidly rotating cool main sequence star - I. Circumstellar clouds

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Abstract
We present time-series of high-resolution AAT CCD spectra of the Hα profile in the active, rapidly rotating G8-K0 dwarf AB Doradus (= HD 36705). These spectra, with time resolution between 180 and 400 s and signal-to-noise ratios of over 100:1, show transient absorption features whose radial velocities relative to the underlying stellar spectrum increase monotonically with time during the course of their 1–2 h lifetimes. Line profile analysis using a simple cloud model suggests that the features originate in cool, dense clouds embedded in and corotating with the hot extended corona. Examinations of these clouds indicate that they preferentially form between 3 and 4 stellar radii from the stellar rotation axis and range in projected area between 3 and 20 per cent of the stellar surface area. They have temperatures in the range ∼ 4500 to 14 000 K and densities which could be as low as 109 cm−3 or as high as 1013 cm−3. The clouds are forced to corotate with the star and thus indicate the presence of closed magnetic loops to very great heights in the stellar atmosphere. Comparison of the structure of these loops with theoretical models suggests that cool, dense clouds could form through thermal instabilities at the summits of such large loops, provided a suitable support mechanism is present. We suggest that this support is supplied by centrifugal forces associated with the rapid stellar rotation. Since the cloud densities may be as much as 107 times greater than would be expected at these heights in normal K0 dwarfs they could possess significant angular momentum. Being relatively cool, the clouds will only be partially ionized, with the neutral material tied to the fieldlines only through collisions. Further, since the clouds are formed beyond the Keplerian corotation radius there will be a net outward force on the material which will eventually lead to a substantial loss of mass and angular momentum. Simple calculations using realistic estimates of cloud densities and volumes suggests that this angular momentum loss could account for rotational braking with time-scales between 107 and 108 yr.

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