Widespread Atomic Gas Emission Reveals the Rotation of the β Pictoris Disk^*

We present high-resolution Na I D spectroscopy of the β Pictoris disk, and the resonantly scattered sodium emission can be traced from less than 30 AU to at least 140 AU from the central star. This atomic gas is coexistent with the dust particles, suggestive of a common origin or source. The disk rotates toward us in the southwest and away from us in the northeast. The velocity pattern of the gas finally provides direct evidence that the faint linear feature seen in images of the star is a circumstellar disk in Keplerian rotation. From modeling the spatial distribution of the Na I line profiles, we determine the effective dynamical mass to be 1.40 ± 0.05 M_☉, which is smaller than the stellar mass, 1.75 M_☉. We ascribe this difference to the gravity opposing the radiation pressure in the Na I lines. We argue that this is consistent with the fact that Na is nearly completely ionized throughout the disk (Na I/Na < 10^-4). The total column density of sodium gas is N(Na) = 10¹⁵ cm^-2.