AAS 203rd Meeting, January 2004
Session 49 Stars Looking Forward to Retirement
Poster, Tuesday, January 6, 2004, 9:20am-6:30pm, Grand Hall

## [49.09] Processing of presolar grains around post-AGB stars: SiC as the carrier of the 21''\mum feature

A. M. Hofmeister (Washington University, St Louis), A. K. Speck (University of Missouri - Columbia)

Intermediate mass stars (0.8-8.0 Msolar) eventually evolve on the H-R diagram, up the asymptotic giant branch (AGB). The intensive mass loss which characterizes the AGB produces a circumstellar shell of dust and neutral gas. At the end of the AGB, mass loss virtually stops and the circumstellar shell begins to drift away from the star. At the same time the central star begins to shrink and heat up. This is the proto-planetary nebula (PPN) phase. Some PPNe exhibit an enigmatic feature in their infrared (IR) spectra at ~21\mum. This feature is not seen in the spectra of either the precursors to PPNe, the AGB stars, or the successors of PPNe, normal'' planetary nebulae (PNe). However the 21''\mum feature has been seen in the spectra of PNe with Wolf-Rayet central stars. Therefore the carrier of this feature is unlikely to be a transient species that only exists in the PPNe phase. This feature has been attributed to various molecular and solid state species, none of which satisfy all constraints, although titanium carbide (TiC) and polycyclic aromatic hydrocarbons (PAHs) have seemed the most viable.

We present new laboratory data for silicon carbide (SiC) and show that it has a spectral feature which is a good candidate for the carrier of the 21\mum feature. The SiC spectral feature appears at approximately the same wavelength (depending on polytype/grain size) and has the same asymmetric profile as the observed astronomical feature. We suggest that processing and cooling of the SiC grains known to exist around carbon-rich AGB stars are responsible for the emergence of the enigmatic 21\mum feature. The emergence of this feature in the spectra of post-AGB stars demonstrates the processing of dust due to the changing physical environments around evolving stars.