Axially symmetric superwinds of proto-planetary nebulae with 21 micron dust features

We present narrowband images at selected wavelengths in the 8-13 mu m window of four carbon-rich, proto-planetary nebulae that have an unusual 21 mu m dust feature: IRAS 04296+3429, IRAS 22272+5435, IRAS 07134+1005, and IRAS 19500-1709. We observe axially symmetric dust emission structures in IRAS 22272+5435 and IRAS 07134+1005, and tentatively in IRAS 19500-1709, while IRAS 04296+3429 is unresolved with our similar to 1'' angular resolution. The well-resolved morphology of IRAS 07134+1005 shows an elliptical outer shell surrounding two aligned peaks that we interpret as limb-brightened peaks of an optically thin, elliptical shell with an equatorial density enhancement. This mid-IR morphology contrasts with that observed in the better studied carbon-rich proto-planetary nebulae, AFGL 2688, AFGL 915, and AFGL 618, which show bright, unresolved cores, probably created by optically thick inner regions, and bipolar extensions that align with their optical reflection nebulosities. Using an axially symmetric dust code and assuming that the dust is composed of 0.01 mu m amorphous carbon grains, we model the dust emission images and the spectral energy distributions of these four proto-planetary nebulae and of the young, carbon-rich planetary nebula IRAS 21282+5050, which also has an axially symmetric dust shell and other similarities with the proto-planetary nebulae that have the 21 mu m dust feature. Marginally resolved mid-infrared images constrain the dust shell's inner radius, while well-resolved mid-infrared images additionally constrain other geometric parameters of the model (e.g., inclination angles and pole-to-equator mass-loss rate ratios). The modeling reveals that the observed axial symmetry in the dust shells of these objects coincides with an enhanced mass-loss phase (similar to 3 x 10(-5) M. yr(-1)) during which the equatorial mass-loss rate was a factor of 18-90 higher than the polar mass-loss rate, i.e., an axially symmetric superwind. Our dynamical age estimates indicate that these stars left the asymptotic giant branch approximately 300-1400 years ago, just after the superwind phase. For each object, the size and structure of the dust shell is the same for the sampled wavelengths, with the exception of IRAS 22272+5435 for which the 11.8 mu m emission is larger than either the 8.2 or the 9.7 mu m emission. IRAS 22272+5435's spectrum has a larger dust feature-to-dust continuum ratio than found in the other objects, and hence its 11.8 mu m image is probably dominated by the 11.8 mu m feature emission that has different optical properties than the underlying continuum.