HST-GHRS observations of β Pictoris: additional evidence for infalling comets

Variable redshifted absorption features observed in the spectrum of β Pictoris have been attributed to comet-like bodies falling towards the star. The high quality data obtained in December 1992 at the unprecedented resolution and sensitivity of the GHRS reinforces the model that both the changes in spectral line shapes as well as the very presence of CI, CIV and CO observed in the environment of β Pic may be explained by the evaporation of several kilometer-sized bodies as they approach the star. Because the spectrum of β Pic varied substantially at the time the observations were done, and because the number of available lines in the UV is much larger than in the visible, it is possible to strongly confirm the proposed scenario. The observations clearly reveal and confirm three types of absorption features presenting different dynamical behaviour: i) a stable one, at the stellar radial velocity and corresponding to a permanent gaseous cloud around the star, ii) slowly varying ones, at modest shifts of few tens km/s which should be related to the evolution of the plasma near the star after its ejection away from the infalling bodies, and iii) the ones presenting rapid changes at highly redshifted velocities directly produced by the infalling and evaporating bodies. The stable as well as the slowly varying features represent a puzzle. A model for the origin of these structures is discussed.