High latitude gas in the β Pictoris system^*

A possible origin related to falling evaporating bodies

Laboratoire d'Astrophysique de Grenoble, UMR 5571 CNRS, Université J. Fourier, BP 53, 38041 Grenoble Cedex 9, France e-mail: Herve.Beust@obs.ujf-grenoble.fr

Received: 13 May 2005
Accepted: 15 December 2006

Abstract

Context.The puzzling detection of  ions at fairly high latitude () above the outer parts of the β Pictoris circumstellar disk was recently reported. Surprisingly, this detection does not extend to  atoms, in contradiction with our modelling of the emission lines in and out of the mid-plane of the disk.

Aims.We propose that the presence of these off-plane  ions (and to a lesser extent  atoms), and the non-detection of off-plane  atoms, could be the consequence of the evaporation process of Falling Evaporating Bodies (FEBs), i.e., star-grazing planetesimals that evaporate in the immediate vicinity of the star.

Methods.Our model is two-fold. Firstly, we show numerically and theoretically that in the star-grazing regime, the FEBs are subject to inclination oscillations up to 30–, and that most metallic species released during each FEB sublimation keep track of their initial orbital inclination while starting a free expansion away from the star, blown out by a strong radiation pressure. Secondly, the off-plane  and  species must be stopped prior to their detection at rest with respect to the star, about 100 AU away. We revisit the role of energetic collisional processes, and we investigate the possible influence of magnetic interactions.

Results.This dynamical process of inclination oscillations explains the presence of off-plane  (and ). It also accounts for the absence of   because once released by the FEBs, these atoms are quickly photoionized and no longer undergo any significant radiation pressure. Our numerical simulations demonstrate that the deceleration of metallic ions can be achieved very efficiently if the ions encounter a dilute neutral gaseous medium. The required  column density is reduced to ~, one order of magnitude below present detection limits. We also investigate the possibility that the ions are slowed down magnetically. While the sole action of a magnetic field of the order of G is not effective, the combined effect of magnetic and collisional deceleration processes lead to an additional lowering of the required  column density by one order of magnitude.