Letter to the Editor

Localizing plages on BO Mic

Rapid variability and rotational modulation of stellar Ca H&K core emission

Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: [uwolter;jschmitt]@hs.uni-hamburg.de

Received: 28 February 2005
Accepted: 31 March 2005

Abstract

We have obtained a densely sampled time series of Ca ii H&K line profiles of the ultrafast rotating K-dwarf star BO Mic. Taken at high resolution, the spectra reveal pronounced variations of the emission core profiles. We interpret these variations as signs of concentrated chromospherically active regions, in analogy to solar plages. We further interpret the variations as partly due to the rapid growth and decay of plages, while other variations appear to be caused by plages moved over the visible stellar disk by rotation. The equivalent width of the Ca K core emission changes approximately in anti-phase to the photospheric brightness, suggesting an association of the chromospheric plage regions with pronounced dark photospheric spots. We believe that further analysis of the presented spectral time series will lead to a chromospheric Doppler image of BO Mic.