Exocomets in the circumstellar gas disk of HD 172555
The source HD 172555 is a young A7V star surrounded by a debris disk with a gaseous component. Here, we present the detection of variable absorption features detected simultaneously in the Ca II K and H doublet lines (at λ3933 A and λ3968 A). We identified the presence of these absorption signatures at four different epochs in the 129 HARPS high-resolution spectra gathered between 2004 and 2011. These transient absorption features are most likely due to falling evaporating bodies (FEBs, or exocomets) that produce absorbing gas observed transiting in front of the central star. We also detect a stable Ca II absorption component at the star’s radial velocity. With no corresponding detection in the Na I line, the resulting very low upper limit for the Na I/Ca II ratio suggests that this absorption is due to circumstellar gas.