Starspot patterns on the M dwarfs HK Aqr and RE 1816 +541

Abstract

We present the first Doppler images of two M dwarfs in the solar neighbourhood. Late-type dwarfs are generally too faint to enable spectra with sufficient signal-to-noise (S/N) ratios for Doppler imaging to be obtained in short enough exposure times. Using the large number of photospheric absorption lines in each spectrum, we apply least-squares deconvolution to derive composite line profiles with S/N ratios of 100–500. The surface images of HK Aqr reveal a moderately spotted surface, with the majority of starspots appearing at low latitudes, whereas RE 1816 +541 exhibits spots at all latitudes. Neither star shows significant evidence for the oft-observed polar spot. Despite the boost in S/N ratio achieved with the least-squares deconvolution technique, the images are not consistent enough to determine reliably the magnitude of differential rotation. Nevertheless, the images are consistent enough to place upper limits on the rate of surface shear.