2018MNRAS.474.1515W

We present high spectral resolution (R ∼ 110 000) Ca II K-line (3933 Å) absorption profiles observed towards 20 nearby A-type stars thought to possess circumstellar debris discs. The stars were repeatedly observed over three nights on the 3.6m ESO telescope (Chile) during 2016 November. These spectra have revealed four stars (HD 24966, HD 38056, HD 79469 and HD 225200) that exhibit transient and weak absorption features at velocities in excess of 15 km s^–1 from the cirumstellar radial velocity. These absorption features mostly appear at redshifted velocities with respect to the stellar radial velocity, some evolving towards bluer velocities as a function of time. We identify these absorption events with the evaporation of gas liberated during the infall of star grazing exocomets/ planetesimals, a process often termed that of 'falling evaporating bodies' (FEBs) (Beust et al. 1990). Five of the stars (HD 2262, HD 30422, HD 45557, HD 181296 and HD 188228) exhibit weak absorption features in at least one of their nightly spectra. However, none of these targets show any significant absorption centred near the stellar radial velocity that could be confidently associated with a circumstellar gas disc. The origin of these sporadic absorptions is unclear at present. Finally, we present a list of 22 stars with known FEB absorption components in their Ca II K-line spectra that will hopefully prove useful as a target list for the possible detection, by high contrast imaging, of exoplanets in orbit around young A-type stars.