The β Pictoris circumstellar disk XXIV . Clues to the origin of the stable gas ?

GHRS high resolution spectra of β Pictoris were obtained to study the stable gas around this star. Several elements are detected and their abundances measured. Upper limits to the abundances of others are also measured. The data permit improved chemical analysis of the stable gas around β Picto-ris, and yield new and more accurate estimates of the radiation pressure acting on various elements. We first analyze the data in the framework of a closed-box model. The electron density is evaluated (N e 10 6 cm −3), which in turn implies constraints on the ionization stages of the various elements. The refractory elements in the stable gas have then standard abundances. In contrast, in this model, the lighter elements sulfur and carbon, observed in their neutral form, seem to be depleted. However several arguments, especially the strong radiation pressure, argue against a closed-box hypothesis. We therefore develop hydrodynamical simulations, taking into account the radiation pressure, to reproduce the stable features under three different hypotheses for the origin of the stable gas: stellar ejection, comet evaporation and grain evaporation. They show that a permanent production of gas is needed in order to sustain a stable absorption. In order to reproduce the observed zero velocity of the absorption features a mechanism is also needed to slow down the radial flow of matter. We show that this could be achieved by a colliding ring of neutral hydrogen farther than 0.5 AU from the star. Applied to the Fe ii lines, the simulations constrain the temperature (T 1 500–2 000 K) and the velocity disper-Send offprint requests to: A.-M. Lagrange Based on observations collected with the Hubble Space Telescope sion (2 km s −1) in the gaseous medium. When applied to Ca ii and to other UV lines, they test the chemical composition of the parent source of gas, which is found to have standard abundances in refractory elements. The gas production rate is 10 −16 M yr −1. This description is the first consistent explanation for these long-lived stable absorptions observed for a large number of lines arising from a variety of energy levels in different chemical elements. It raises the question of the origin of the parent material, together with its composition and dynamics. This realizes a link between this gaseous component and the whole circumstellar system.