AXIALLY-SYMMETRICAL SUPERWINDS OF PROTOPLANETARY 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 unusu al 21 mu m dust feature: IRAS 04296+3429, IRAS 22272+5435, IRAS 071341005, 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 071341005 shows an elliptical outer shell surrounding two aligned peaks tha t we interpret as limb-brightened peaks of an optically thin, elliptic al shell with an equatorial density enhancement. This mid-IR morpholog y contrasts with that observed in the better studied carbon-rich proto -planetary nebulae, AFGL 2688, AFGL 915, and AFGL 618, which show brig ht, unresolved cores, probably created by optically thick inner region s, and bipolar extensions that align with their optical reflection neb ulosities. Using an axially symmetric dust code and assuming that the dust is composed of 0.01 mu m amorphous carbon grains, we model the du st emission images and the spectral energy distributions of these four proto-planetary nebulae and of the young, carbon-rich planetary nebul a IRAS 21282+5050, which also has an axially symmetric dust shell and other similarities with the proto-planetary nebulae that have the 21 m u m dust feature. Marginally resolved mid-infrared images constrain th e dust shell's inner radius, while well-resolved mid-infrared images a dditionally constrain other geometric parameters of the model (e.g., i nclination angles and pole-to-equator mass-loss rate ratios). The mode ling reveals that the observed axial symmetry in the dust shells of th ese 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 fa ctor of 18-90 higher than the polar mass-loss rate, i.e., an axially s ymmetric superwind. Our dynamical age estimates indicate that these st ars left the asymptotic giant branch approximately 300-1400 years ago, just after the superwind phase. For each object, the size and structu re 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 larg er than either the 8.2 or the 9.7 mu m emission. IRAS 22272+5435's spe ctrum 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 properti es than the underlying continuum.