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