  Ever since the discovery of the edge-on circumstellar disk around beta
Pictoris, a standing question has been why the gas observed against the star in
absorption is not rapidly expelled by the strong radiation pressure from the
star. A solution to the puzzle has been suggested to be that the neutral
elements that experience the radiation force also are rapidly ionized, and so
are only able to accelerate to an average limiting velocity v_ion. Once
ionized, the elements are rapidly braked by C II, which is observed to be at
least 20x overabundant in the disk with respect to other species. A prediction
from this scenario is that different neutral elements should reach different
v_ion, depending on the ionization thresholds and strengths of driving line
transitions. In particular, neutral Fe and Na are predicted to reach the radial
velocities 0.5 and 3.3 km/s, respectively, before being ionized. In this paper
we study the absorption profiles of Fe and Na from the circumstellar gas disk
around beta Pic, as obtained by HARPS at the ESO 3.6m telescope. We find that
the Fe and Na velocity profiles are indeed shifted with respect to each other,
confirming the model. The absence of an extended blue wing in the profile of
Na, however, indicates that there must be some additional braking on the
neutrals. We explore the possibility that the ion gas (dominated by C II) can
brake the neutrals, and conclude that about 2-5x more C than previously
estimated is needed for the predicted line profile to be consistent with the
observed one.
