Deuterated polycyclic aromatic hydrocarbons: Revisited

TL;DR

This study uses near-infrared spectroscopy of HII regions across different galaxies to detect deuterated PAHs, revealing that PAH deuteration is rare in the interstellar medium, with only a few confirmed cases.

Contribution

It provides the first systematic search for deuterated PAHs in multiple galaxies using AKARI NIR spectra, and constrains the D/H ratio in PAHs.

Findings

Deuterated PAH features detected in 6 Milky Way sources.

Weak or absent deuterated PAH features in most observed regions.

Deuteration of PAHs in the ISM appears to be an uncommon process.

Abstract

The amount of deuterium locked up in polycyclic aromatic hydrocarbons (PAHs) has to date been an uncertain value. We present a near-infrared (NIR) spectroscopic survey of HII regions in the Milky Way, Large Magellanic Cloud (LMC), and Small Magellanic Cloud (SMC) obtained with AKARI, which aims to search for features indicative of deuterated PAHs (PAD or Dn-PAH) to better constrain the D/H ratio of PAHs. Fifty-three HII regions were observed in the NIR (2.5-5 {\mu}m), using the Infrared Camera (IRC) on board the AKARI satellite. Through comparison of the observed spectra with a theoretical model of deuterated PAH vibrational modes, the aromatic and (a)symmetric aliphatic C-D stretch modes were identified. We see emission features between 4.4-4.8 {\mu}m, which could be unambiguously attributed to deuterated PAHs in only six of the observed sources, all of which are located in the Milky Way. In all cases, the aromatic C-D stretching feature is weaker than the aliphatic C-D stretching feature, and, in the case of M17b, this feature is not observed at all. Based on the weak or absent PAD features in most of the observed spectra, it is suggested that the mechanism for PAH deuteration in the ISM is uncommon.