On Iron Monoxide Nanoparticles as a Carrier of the Mysterious 21
  Micrometer Emission Feature in Post-Asymptotic Giant Branch Stars
  A prominent, mysterious emission feature peaking at ~20.1 micrometer ---
historically known as the ``21 micrometer' feature --- is seen in over two
dozen Galactic and Magellanic Cloud carbon-rich post-asymptotic giant branch
(post-AGB) stars. The nature of its carrier remains unknown since the first
detection of the 21 micrometer feature in 1989. Over a dozen materials have
been suggested as possible carrier candidates. However, none of them has been
accepted: they either require too much material (compared to what is available
in the circumstellar shells around these post-AGB stars), or exhibit additional
emission features which are not seen in these 21 micrometer sources.
  Recently, iron monoxide (FeO) nanoparticles seem to be a promising carrier
candidate as Fe is an abundant element and FeO emits exclusively at ~21
micrometer. In this work, using the proto-typical protoplanetary nebula HD
56126 as a test case, we examine FeO nanoparticles as a carrier for the 21
micrometer feature by modeling their infrared emission, with FeO being
stochastically heated by single stellar photons. We find that FeO emits too
broad a 21 micrometer feature to explain the observed one and the Fe abundance
required to be locked up in FeO exceeds what is available in HD 56126. We
therefore conclude that FeO nanoparticles are unlikely responsible for the 21
micrometer feature.
