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

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 hi gh signal-to-noise ratio profiles from 300-s exposures, we apply the techni que of least-squares deconvolution to the large number of photospheric abso rption lines available in each of our spectra. This allows us to derive hig h-resolution maximum-entropy-regularized Doppler images of the stellar surf ace. Using these techniques, we also derive radial velocities and accurate projected equatorial rotation velocities which are consistent to within sim ilar to1 km s(-1) Our surface maps reveal one of the most heavily spotted photospheres seen o n a rapid rotator, with starspots occurring at all latitudes, At the time o f observations, Speedy Mic had no strong polar spot, but it shows spots con centrated in low- and intermediate-latitude bands. We attempt a differentia l rotation measurement, but lack of sufficient phase coverage allows determ ination 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 attr ibuted to prominences passing in front of the stellar disc. Despite the rap id rotation, the appearance of the same features on consecutive nights of o bservations shows the clouds to be stable on time-scales of at least a day.