%0 Journal Article %T Subarcsecond Mid-IR Structure of the Dust Shell around IRAS 22272+5435 %A T. Ueta %A M. Meixner %A P. M. Hinz %A W. F. Hoffmann %A W. Brandner %A A. Dayal %A L. K. Deutsch %A G. G. Fazio %A J. L. Hora %J Physics %D 2001 %I arXiv %R 10.1086/322259 %X We report sub-arcsecond imaging of extended mid-infrared 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-infrared emission structure is resolved into two emission peaks separated by $0\arcsec.5 - 0\arcsec.6$ in the MMT 11.7 $\um$ image and in the Keck 7.9, 9.7, and 12.5 $\um$ images, corroborating the predictions based on previous multi-wavelength morphological studies and radiative transfer calculations. The resolved images show that the PPN dust shell has a toroidal structure with the $0\arcsec.5$ inner radius. In addition, an unresolved mid-IR excess appears at the nebula center. Radiative transfer model calculations suggest that the highly equatorially-enhanced ($\rho_{\rm eq}/\rho_{\rm pole} = 9$) structure of the PPN shell was generated by an axisymmetric superwind with ${\dot M}_{\rm sw} = 4 \times 10^{-6} M_{\odot}$ yr$^{-1}$, which was preceded by a spherical asymptotic giant branch (AGB) wind with ${\dot M}_{\rm AGB} = 8 \times 10^{-7} M_{\odot}$ yr$^{-1}$. These model calculations also indicate that the superwind shell contains larger dust grains than the AGB wind shell. The unresolved mid-infrared excess may have been produced by a post-AGB mass loss at a rate of $2 - 6 \times 10^{-7} M_{\odot}$ yr$^{-1}$. While the central star left the AGB about 380 years 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 years ago. %U http://arxiv.org/abs/astro-ph/0104437v1