Abstract
Abstract Kiefer et al. reported the detection of infalling Ca ii absorption in HD 172555, a member of the β Pictoris Moving Group (βPMG). 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 β 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.
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