Reconsidering the Origin of the 21 Micron Feature: Oxides in Carbon‐rich Protoplanetary Nebulae?

Abstract

The origin of the so-called '21' µm feature which is especially prominent in the spectra of some carbon-rich protoplanetary nebulae (PPNe) is the matter of a lively debate. A large number of potential band carriers have been presented and discarded within the past decade. The present paper gives an overview of the problems related to the hitherto proposed feature identifications, including the recently suggested candidate carrier silicon carbide. We also discuss the case for spectroscopically promising oxides.SiC is shown to produce a strong resonance band at 20-21 µm if coated by a layer of silicon dioxide. At low temperatures, core-mantle particles composed of SiC and amorphous SiO 2 indeed have their strongest spectral signature at a position of 20.1 µm, which coincides with the position of the '21' µm emission band.The optical constants of another candidate carrier that has been relatively neglected so far -iron monoxide -are proven to permit a fairly accurate reproduction of the '21' µm feature profile as well, especially when low-temperature measurements of the infrared properties of FeO are taken into account. As candidate carrier of the '21' µm emission band, FeO has the advantage of being stable against further oxidation and reduction only in a narrow range of chemical and physical conditions, coinciding with the fact that the feature, too, is detected in a small group of objects only. However, it is unclear how FeO should form or survive particularly in carbon-rich PPNe.

Organic matter in space: from star dust to the Solar System

“…Since the AIB emission features are also seen in the 21 µm sources (see Table 1), it is likely that the carrier of the 21 µm feature is related to aromatic ring carbon compounds, or even larger molecules that might form from such compounds. Possible candidates that have been proposed include large PAH clusters, HAC grains, hydrogenated fullerenes, nanodiamonds, TiC nanoclusters, nanoSiC grains with carbon impurities, and cold SiC grains with amorphous SiO 2 mantles (see Posch et al 2004;Speck and Holmeister 2004 and references therein). The unidentified emission feature around 30 µm was discovered from KAO observations (Forrest et al 1981).…”

Unidentified Species in Envelopes around Carbon Stars

“…However, it is not clear how FeO forms in C-rich environments as the 21 μm sources are all C-rich. Posch et al (2004) argued that FeO nanodust could form in these environments. We therefore model the excitation and emission processes of FeO nanodust in PPNe.…”

Synthesis of organic compounds in the circumstellar environment

“…Now over a dozen PPNe (but not AGB stars and PNe) have been seen to possess this feature (Hrivnak et al, these proceedings). Many different candidates have been proposed as the carrier of this feature, including the recent proposals of SiC (Speck & Hofmeister 2004) and FeO (Posch et al 2004).…”