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

Original · EN

We report sub-arcsecond imaging of extended mid-infrared emission from a proto-planetary nebula (PPN), 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 0.5 - 0.6 in the MMT 11.7 image and in the Keck 7.9, 9.7, and 12.5 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 0.5 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 yr⁻¹, which was preceded by a spherical asymptotic giant branch (AGB) wind with M 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-infrared 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 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.