Braking the Gas in the Β Pictoris Disk

Metallic gas detected in the β Pictoris circumstellar debris disk raises many questions. The origin of this gas is unclear and its very presence is difficult to explain: many constituents of the gas are expected to be radiatively accelerated outward, yet their motion appears to be consistent with Keplerian rotation out to at least 300 AU. Hydrogen has previously been hypothesized to exist in the disk, acting as a braking agent, but the amount required to brake individual elements conflicted with observed upper limits. To resolve this discrepancy, we search for alternative braking mechanisms for the metallic gas. We find that all species affected by radiation force are heavily ionized. Frequent Coulomb collisions couple the ions into a single fluid, reducing the radiation force on species feeling the strongest acceleration. For a gas of solar composition, the resulting total radiation force still exceeds gravity, while a gas of enhanced carbon abundance could be self-braking. We also explore two other braking agents: collisions with dust grains and neutral gas. Grains surrounding β Pic are photoelectrically charged to a positive electrostatic potential. If a significant fraction of the dust grains are carbonaceous (10% in the midplane and larger at higher altitudes), ions can be slowed down to satisfy the observed velocity constraints. In this case, both the gas kinematics and spatial distributions are expected to coincide with those of small grains, the latter being indeed observed. For neutral gas to brake the coupled ion fluid, we find the minimum required mass to be ≈ 0.03 M⊕, consistent with observed upper limits of the hydrogen column density, and substantially reduced relative to previous estimates. Our results favor a scenario in which metallic gas is generated by grain evaporation in the disk, perhaps during grain-grain collisions. We exclude a primordial origin for the gas, but cannot rule out the possibility of its production by falling evaporating bodies near the star. We discuss the implications of this work for observations of gas in other debris disks. Subject headings: stars: individual (β Pictoris) — circumstellar matter — acceleration of particles — scattering — planetary systems: formation — planetary systems: protoplanetary disks