TIMMI2 diffraction–limited mid-infrared images of a multipolar proto-planetary nebula IRAS 16594 - 4656 and a young [ WC ] elliptical planetary nebula IRAS 07027 - 7934 are presented .
Their dust shells are for the first time resolved ( only marginally in the case of IRAS 07027 - 7934 ) by applying the Lucy-Richardson deconvolution algorithm to the data , taken under exceptionally good seeing conditions ( \leq 0.5″ ) .
IRAS 16594 - 4656 exhibits a two-peaked morphology at 8.6 , 11.5 and 11.7 \mu m which is mainly attributed to emission from PAHs .
Our observations suggest that the central star is surrounded by a toroidal structure observed edge-on with a radius of 0.4 \arcsec ( \sim 640 AU at an assumed distance of 1.6 kpc ) with its polar axis at P.A . \sim 80 ^ { \circ } , coincident with the orientation defined by only one of the bipolar outflows identified in the HST optical images .
We suggest that the material expelled from the central source is currently being collimated in this direction and that the multiple outflow formation has not been coeval .
IRAS 07027 - 7934 shows a bright , marginally extended emission ( FWHM=0.3 \arcsec ) in the mid-infrared with a slightly elongated shape along the N-S direction , consistent with the morphology detected by HST in the near-infrared .
The mid-infrared emission is interpreted as the result of the combined contribution of small , highly ionized PAHs and relatively hot dust continuum .
We propose that IRAS 07027 - 7934 may have recently experienced a thermal pulse ( likely at the end of the AGB ) which has produced a radical change in the chemistry of its central star .