Unidentified Species in Envelopes around Carbon Stars

Abstract

Abstract. The infrared (IR) spectra of many evolved carbon-rich stars exhibit two prominent dust emission features peaking around 21 μm and 30 μm, 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 30 μm feature is seen in all the 21 μm sources, but no correlation is found between these two features. Over a dozen carrier candidates have been proposed for the 21 μm feature, but none of them has been widely accepted and the nature of the 21 μm feature remains a mystery. The carrier of the 30 μm feature also remains unidentified. 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 21 μm feature. We calculate the IR emission spectrum of FeO nanodust which undergoes single-photon heating in PPNe. It is found that the 21 μm feature emitted by FeO nanodust is too broad to explain the observed feature. For the 30 μm feature, we argue that graphite could be a viable carrier. Graphite, provided its d.c. conductivity σ d . c . exceeds ∼100 ohm −1 cm −1 , exhibits a pronounced band at 30 μm.

Chemical abundances in the PN Wray16-423 in the Sagittarius dwarf spheroidal galaxy: constraining the dust composition

“…Another possible candidate for the 30 µm feature is graphite, as proposed by Jiang et al (2014). The opacity of graphite by Draine & Lee (1984) exhibits broad emission (FWHM>60 µm) peaked at ∼30-40 µm.…”

“…The opacity of graphite by Draine & Lee (1984) exhibits broad emission (FWHM>60 µm) peaked at ∼30-40 µm. The theoretical opacity of graphite with a d.c. conductivity of 100 Ω −1 cm −1 by Jiang et al (2014) shows a well-consistent FWHM of ∼10 µm to the observed FWHMs reported by Hony et al (2002a). Otsuka et al (2014) found that the relative strength of the 30 µm feature to the underlying continuum in fullerene C 60 -containing PNe is constant; they also succeeded in fitting both the 30 µm feature and the 13-160 µm continuum using the synthesised absorption efficiency Q abs (λ), derived from the combined data of ISO/SWS/LWS and Spitzer/IRS spectra of C 60 -containing PN IC418.…”

“…If any sulfide molecules form in the nebula, then it would be more difficult to explain the 30 µm feature with Fe50S and MgS. If it is possible to estimate the mass of the S-atom existing as the molecule with sub-mm spectra and the optical characteristics data of MgS, Fe50S and the new graphite data by Jiang et al (2014) are available over a wider wavelength range, then it may be possible to resolve which grains carry the 30 µm feature.…”

Dust Production by Evolved Stars in the Magellanic Clouds

“…The continuum could be (partially) due to graphite, rather than amorphous carbon, and graphite has a resonance at approximately the right wavelength (Draine & Lee, 1984;Jiang et al, 2013). Thus, a separate dust component to explain the "30 µm" feature may no longer be needed.…”