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M. Meixner, A.K. Speck, T. Ueta (University of Illinois), G.R. Knapp (Princeton University), W.F. Hoffmann, P.M. Hinz (University of Arizona), J.L. Hora, G. Fazio (CFA/ Harvard), L.K Deutsch (Boston University)
We present recent results from ISOPHOT C200 array and ground based mid-infrared imaging of several proto-planetary nebulae (PPNe), which are evolved objects in transition from the asymptotic giant branch (AGB) star to the planetary nebula (PN) phase of stellar evolution. These infrared data are sensitive to the thermal emission from the circumstellar dust in these objects. With ISOPHOT AOT PHT32, we discovered parsec-sized dust shells around AFGL 2688 and AFGL 618, which are the two best studied carbon-rich PPNe. The 53'\times3' long linear scans of 120 and 180 \mum emission from AFGL 2688 and AFGL 618 are characterized by very bright central point sources and low-level (10% of the peak) extended emission out to approximately 400" in radius. Interestingly, the radial intensity distribution is not smooth, but shows ``periodic'' enhancements of emission at approximately 150" and 300". Assuming a distance and velocity, we derive timescales of a few 104 yrs between events which are comparable to the theoretical thermal pulse time scales on the AGB. In contrast to the far-infrared dust emission observed with ISOPHOT, ground-based mid-infrared imaging has better angular resolution to see the interior structure of the circumstellar dust shell, but it is only senstive to the warm (~100 K) dust located in the inner regions. With the MIRAC3 ground-based imaging camera on the MMT telescope, we have imaged the PPN, IRAS 22272+5435 at 8.8, 9.8, and 11.7 \mu m. At 11.7 \mu m, we find a ring of dust emission and the spatial extent of this ring coincides with the reflection nebula imaged with HST. We present radiative transfer calculations of the PPN IRAS 22272+5435 to interpret the results.
Acknowledgments: The ISO work by Speck, Meixner and Knapp has been supported by NASA (NAG 5-3350 and JPL 961504). The mid-infrared imaging by Meixner, Ueta and the MIRAC3 team has been partially supported by NSF (AST-97-33697).