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

Abstract

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 0.5"-0.6" 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 0.5" inner radius. In addition, an unresolved mid-IR excess appears at the nebula center. Radiative transfer model calculations suggest that the highly equatorially enhanced (ρ_eq/ρ_pole=9) structure of the PPN shell was generated by an axisymmetric superwind with M_sw=4×10^-6 M_solar yr^-1, which was preceded by a spherical asymptotic giant branch (AGB) wind with M_AGB=8×10^-7 M_solar 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_solar 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. The observational data presented here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution, and at the W. M. Keck Observatory, which was made possible by the generous financial support of the W. M. Keck Foundation and which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration.