On Iron Monoxide Nanoparticles as a Carrier of the Mysterious 21 Micrometer Emission Feature in Post-Asymptotic Giant Branch Stars

TL;DR

This study investigates iron monoxide nanoparticles as a potential carrier of the mysterious 21 micrometer emission feature in post-AGB stars, but finds they are unlikely responsible due to emission broadness and Fe abundance constraints.

Contribution

The paper evaluates FeO nanoparticles as a candidate for the 21 micrometer feature using emission modeling, providing evidence against their role in this astronomical phenomenon.

Findings

FeO emits too broad a 21 micrometer feature

Fe abundance needed exceeds available in HD 56126

FeO unlikely to be the carrier of the 21 micrometer feature

Abstract

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.