The Star-grazing Bodies in the HD 172555 System

Kiefer et al. reported the detection of infalling Ca II absorption in HD 172555, a member of the beta Pictoris Moving Group (beta PMG). We obtained HST Space Telescope Imaging Spectrograph and Cosmic Origins Spectrograph spectroscopy of this star at 2 epochs separated by a week, and we report the discovery of infalling gas in resonant transitions of Si III and IV, C II and IV, and neutral atomic oxygen. Variable absorption is seen in the C II transitions and is optically thick, with covering factors which range between 58% and 68%, similar to features seen in beta Pictoris. The O I spectral profile resembles that of C II, showing a strong low-velocity absorption to + 50 km s(-1) in the single spectral segment obtained during orbital night, as well as what may be higher-velocity absorption. Studies of the mid-IR spectrum of this system have suggested the presence of silica. The O I absorption differs from that seen in Si III, suggesting that the neutral atomic oxygen does not originate in SiO dissociation products but in a more volatile parent molecule such as CO.