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Session 15 - Planetary Nebulae and White Dwarfs.
Display session, Wednesday, January 07
Exhibit Hall,
We are engaged in a multi-wavelength program to identify and study proto-planetary nebulae (PPNe), objects in transition between the AGB and PN phases. These objects are characterized by strong infrared emission from dust in an optically-thin circumstellar shell, resulting in a double-peaked spectral energy distribution (reddened photosphere plus circumstellar dust). We report here the identification of two unusual PPNe, IRAS 02229+6208 and 07430+1115. (Note that 02229+6208 is not found in the IRAS Point Source Catalogue, but was found from a color-selected study of the Faint Source Reject file.) Both objects display C2 and C3 absorption in their optical spectra. This brings to 12 the number of PPNe in which molecular carbon has been found (see Hrivnak 1995, ApJ, 438, 341). Both have spectra that resemble those of G supergiants, consistent with the expectation of their being PPNe. Ground-based optical and infrared photometry from KPNO shows double-peaked spectral energy distributions, suggesting that the AGB mass loss has ended and that these objects are in the post-AGB phase of evolution. The remnant of the molecular envelope is detected in CO(3-2) emission in 02229+6208. The 3.3 and 11.3 \mum emission features commonly attributed to PAH molecules have been detected in 07430+1115 with spectra obtained at UKIRT. Strikingly absent in 07430+1115, however, is a 21 \mum emission feature seen in the other PPNe which have C2. This is surprising, since a correlation has been found between the presence of C2 absorption, 3.3 \mum emission, and 21 \mum emission (Kwok, Hrivnak, amp; Geballe 1995, ApJ, 454, 394). A recent ISO spectrum of 02229+6208 reveals the presence of the 11.3 and 21 \mum emission features in this object. The origin of the 21 \mum feature is still unknown, but the absence of it in 07430+1115 may provide a key to the conditions under which it forms.
