THE BETA-PICTORIS CIRCUMSTELLAR DISK XXIV - CLUES TO THE ORIGIN OF THE STABLE GAS

GHRS high resolution spectra of beta Pictoris were obtained to study t he stable gas around this star. Several elements are detected and thei r abundances measured. Upper limits to the abundances of others are al so measured. The data permit improved chemical analysis of the stable gas around beta Pictoris, and yield new and more accurate estimates of the radiation pressure acting on various elements. We first:analyze t he data in the framework of a closed-box model. The electron density i s evaluated (N-e similar or equal to 10(6) cm(-3)), which in turn impl ies constraints on the ionization stages of the various elements. The refractory elements in the stable gas have then standard abundances. I n contrast, in this model, the lighter elements sulfur and carbon, obs erved in their neutral form, seem to be depleted. However several argu ments, especially the strong radiation pressure, argue against a close d-box hypothesis. We therefore develop hydrodynamical simulations, tak ing into account the radiation pressure, to reproduce the stable featu res 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 sta ble absorption. In order to reproduce the observed zero velocity of th e absorption features a mechanism is also needed to slow down the radi al 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 FeII lines, the simulations constrain the temperature (T similar or equal to 1500-2000K) and the velocity dispersion (similar or equal to 2 km s(-1)) in the gaseous medium. When applied to CaII and to othe r UV lines, they test the chemical composition of the parent source of gas, which is found to have standard abundances in refractory element s. The gas production rate is similar or equal to 10(-16) M. yr(-1). T his description is the first consistent explanation for these long-liv ed stable absorptions observed for a large number of lines arising fro m 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 co mponent and the whole circumstellar system.