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Unidentified species in envelopes around carbon stars

The infrared (IR) spectra of many evolved carbon-rich stars exhibit two prominent dust emission features peaking around 21um and 30um, with the former exclusively seen in proto-planetary nebulae (PPNe), while the latter seen in a much wider range of objects, including AGB stars, PPNe and planetary nebulae (PNe). The 30um feature is seen in all the 21um sources, but no correlation is found between these two features. Over a dozen carrier candidates have been proposed for the 21um feature, but none of them has been widely accepted and the nature of the 21um feature remains a mystery. The carrier of the 30um feature also remains unidentied. MgS dust, once widely accepted as a valid carrier, was ruled out because of the sulfur budget problem. In this work we examine nano-sized FeO dust as a carrier for the 21um feature. We calculate the IR emission spectrum of FeO nanodust which undergoes single-photon heating in PPNe. It is found that the 21um feature emitted by FeO nanodust is too broad to explain the observed feature. For the 30um feature, we argue that graphite could be a viable carrier. Graphite, provided its d.c. conductivity sigma_dc exceeding 100/ohm/cm, exhibits a pronounced band at 30um.