Revealing the mid-infrared emission structure of IRAS 16594-4656 and IRAS 07027-7934

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 (< 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 (similar to 640 AU at an assumed distance of 1.6 kpc) and with its polar axis at P.A. similar to 80 degrees, 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) in the mid-infrared with a slightly elongated shape along the north-south 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.