Subarcsecond Mid-Infrared Structure of the Dust Shell around IRAS 22272+5435^*

We report subarcsecond imaging of extended mid-IR emission from a proto-planetary nebula (PPN), IRAS 22272+5435, performed at the MMT observatory with its newly upgraded 6.5 m aperture telescope and at the Keck observatory. The mid-IR emission structure is resolved into two emission peaks separated by 05-06 in the MMT 11.7 μm image and in the Keck 7.9, 9.7, and 12.5 μm images, corroborating the predictions based on previous multiwavelength morphological studies and radiative transfer calculations. The resolved images show that the PPN dust shell has a toroidal structure with the 05 inner radius. In addition, an unresolved mid-IR excess appears at the nebula center. Radiative transfer model calculations suggest that the highly equatorially enhanced (ρ_eq/ρ_pole = 9) structure of the PPN shell was generated by an axisymmetric superwind with _sw = 4 × 10^-6 M_☉ yr^-1, which was preceded by a spherical asymptotic giant branch (AGB) wind with _AGB = 8 × 10^-7 M_☉ yr^-1. These model calculations also indicate that the superwind shell contains larger dust grains than the AGB wind shell. The unresolved mid-IR excess may have been produced by a post-AGB mass loss at a rate of 2-6 × 10^-7 M_☉ yr^-1. While the central star left the AGB about 380 yr ago after the termination of the superwind, the star seems to have been experiencing an ambient post-AGB mass loss with a sudden, increased mass ejection about 10 yr ago.