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 \mu m image and in the Keck 7.9 , 9.7 , and 12.5 \mu m 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 _ { eq } / \rho _ { pole } = 9 ) structure of the PPN shell was generated by an axisymmetric superwind with { \dot { M } } _ { sw } = 4 \times 10 ^ { -6 } M _ { \odot } yr ^ { -1 } , which was preceded by a spherical asymptotic giant branch ( AGB ) wind with { \dot { M } } _ { 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 .