Doppler images from dual-site observations of southern rapidly rotating stars - II. Starspot patterns and differential rotation on Speedy Mic

Abstract

We have secured high spatial and temporal resolution spectra of the rapidly rotating K dwarf Speedy Mic (HD 197890) at two sites and a common epoch of observations. The 0.38-d axial rotation period and the V-band magnitude of 9.33 make it a difficult target for Doppler imaging. In order to obtain high signal-to-noise ratio profiles from 300-s exposures, we apply the technique of least-squares deconvolution to the large number of photospheric absorption lines available in each of our spectra. This allows us to derive high-resolution maximum-entropy-regularized Doppler images of the stellar surface. Using these techniques, we also derive radial velocities and accurate projected equatorial rotation velocities which are consistent to within similar to1 km s(-1) Our surface maps reveal one of the most heavily spotted photospheres seen on a rapid rotator, with starspots occurring at all latitudes, At the time of observations, Speedy Mic had no strong polar spot, but it shows spots concentrated in low- and intermediate-latitude bands. We attempt a differential rotation measurement, but lack of sufficient phase coverage allows determination of only a lower limit of 59 d for the time it takes the equatorial regions to lap the polar regions. We also find variations in the heavily filled-in Her line which can be attributed to prominences passing in front of the stellar disc. Despite the rapid rotation, the appearance of the same features on consecutive nights of observations shows the clouds to be stable on time-scales of at least a day.