The spatial structure of the β Pictoris gas disk

We have used VLT/UVES to spatially resolve the gas disk of beta Pictoris. 88 extended emission lines are observed, with the brightest coming from Fe I, Na I and Ca II. The extent of the gas disk is much larger than previously anticipated; we trace Na I radially from 13 AU out to 323 AU and Ca II to heights of 77AU above the disk plane, both to the limits of our observations. The degree of flaring is significantly larger for the gas disk than the dust disk. A strong NE/SW brightness asymmetry is observed, with the SW emission being abruptly truncated at 150-200 AU. The inner gas disk is tilted about 5. with respect to the outer disk, similar to the appearance of the disk in light scattered from dust. We show that most, perhaps all, of the Na I column density seen in the "stable" component of absorption, comes from the extended disk. Finally, we discuss the effects of radiation pressure in the extended gas disk and show that the assumption of hydrogen, in whatever form, as a braking agent is inconsistent with observations.