Latitude distributions and lifetimes of star-spots on G dwarfs in the alpha Persei cluster

Abstract

We present a set of maximum-entropy reconstructions of the star-spot distributions on two rapidly rotating G dwarfs in the alpha Persei cluster, from spectra taken at the William Herschel Telescope on three nights in 1996 October and November. Since these stars are too faint for conventional Doppler imaging, which makes use of only one or a few lines, we take the large number of photospheric metal lines available in an echelle spectrum, and deconvolve them into a single, high signal-to-noise ratio profile. We show that this technique results in a typical multiplex gain of 22.5 in signal-to-noise ratio for a given spectrum, the equivalent of using a single line obtained on a 63-m telescope. The image reconstructions demonstrate that both these stars have cool high-latitude regions or polar crowns, and low-latitude features, in contradiction to the suggestion that only high-latitude spots should be present. Cross-correlation between image reconstructions of He 699, 31 days apart, reveals a lack of correlation between detailed small-scale structures. This places an upper limit for the lifetime of the observed features at less than one month. The H alpha profiles are also found to exhibit absorption features indicating the presence of prominence clouds, at or below the corotation radius.