Unidentified species in envelopes around carbon stars
Biwei Jiang, Aigen Li, Ke Zhang, Jiaming Liu, Jian Gao, A. Mishra

TL;DR
This study investigates potential carriers for the mysterious 21um and 30um IR emission features in evolved carbon-rich stars, ruling out some candidates and proposing FeO nanodust and graphite as possible carriers.
Contribution
It introduces a detailed analysis of FeO nanodust for the 21um feature and suggests graphite as a viable carrier for the 30um feature, advancing understanding of dust composition in these stars.
Findings
FeO nanodust produces a too broad 21um feature
Graphite with high conductivity can produce a 30um feature
Carrier candidates for the 21um feature remain uncertain
Abstract
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…
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