On the Carriers of the 21 Micron Emission Feature in Post-Asymptotic Giant Branch Stars

TL;DR

This study evaluates various proposed carriers for the 21 micron emission feature in post-AGB stars, using elemental abundance constraints to identify the most plausible candidate among several options.

Contribution

It provides a quantitative assessment of eight inorganic and one organic candidate materials based on elemental abundance constraints, narrowing down the viable carriers.

Findings

Five candidates violate abundance constraints.

Three candidates produce undetected secondary features.

Nano FeO remains a viable carrier candidate.

Abstract

The mysterious 21micron emission feature seen in sixteen C-rich proto-planetary nebulae (PPNe) remains unidentified since its discovery in 1989. Over a dozen of materials are suggested as the carrier candidates. In this work we quantitatively investigate eight inorganic and one organic carrier candidates in terms of elemental abundance constraints, while previous studies mostly focus on their spectral profiles (which could be largely affected by grain size, shape and clustering effects). It is found that: (1) five candidates (TiC nanoclusters, fullerenes coordinated with Ti atoms, SiS$_2$, doped-SiC, and SiO$_2$-coated SiC dust) violate the abundance constraints (i.e. they require too much Ti, S or Si to account for the emission power of the 21micron band, (2) three candidates (carbon and silicon mixtures, Fe$_2$O$_3$, and Fe$_3$O$_4$),while satisfying the abundance constraints, exhibit secondary features which are not detected in the 21micron sources, and (3) nano FeO, neither exceeding the abundance budget nor producing undetected secondary features, seems to be a viable candidate, supporting the suggestions of Posch et al. 2004.