Prominence activity on the rapidly rotating field star HD 197890 *

Abstract

Time-resolved H αspectra are presented of the recently discovered, rapidly rotating young field star HD 197890. The observations contain rapid absorption and emission transients that cross the rotationally broadened Hα profile from blue to red on timescales of order 1 h. The behavior of the absorption transients is consistent with models proposed for another rapidly rotating field star, AB Dor, in which cool clouds of material are trapped in corotating closed magnetic structures above the surface of the star and scatter chromospheric H α photons out of the line of sight. Measurements of the radial acceleration of these clouds indicate that they are positioned at about or outside the Keplerian corotarion radius. Observations on three nights show that clouds may appear and on time-scales of days, and may therefore be a key component of the rapid angular momentum loss experienced by very young stars. The emission transients precede the absorption transients, and it is proposed that these are due to active areas on the stellar surface or are signatures of flaring activity that could be triggered by the presence of cool prominences, and also supply new prominences with material through chromospheric evaporation.