Dust Distribution in Gas Disks: A Model for the Ring around HR 4796A

There have been several model analyses of the near and mid IR flux from thecircumstellar ring around HR4796A. In the vicinity of a young star, thepossibility that the dust ring is embedded within a residual protostellar gasdisk cannot be ruled out. In a gas-rich environment, larger sizes ($>100 \mum$) are needed for the particles to survive the radiative blow out. The totaldust mass required to account for the IR flux is $< 10^{-1} M_\oplus$. Thecombined influence of gas and stellar radiation may also account for theobserved sharp inner boundary and rapidly fading outer boundary of the ring.The pressure gradient induced by a small (10%) amplitude variation in thesurface density distribution of a low-mass gaseous disk would be sufficient tomodify the rotation speed of the gas.Comment: proof read version, 26 pages, LaTex, 11 figures. To appear in The Astronomical Journal June 200