Theoretical study of deuteronated PAHs as carriers for IR emission features in the ISM

TL;DR

This theoretical study investigates deuteronated PAH molecules as carriers for specific IR emission features in the interstellar medium, analyzing their spectral properties and implications for deuterium abundance.

Contribution

It provides a detailed theoretical analysis of DPAH+ molecules' spectral features and their potential role in explaining observed IR emission bands in space.

Findings

DPAH+ molecules show IR features due to C-D bond stretching.

Large DPAH+ molecules match observed D/H ratios in the ISM.

D/H ratio decreases with increasing DPAH+ size.

Abstract

This work proposes deuteronated PAH (DPAH+ ) molecules as a potential carrier of the 4.4 and 4.65 {\mu}m mid infrared emission bands that have been observationally detected towards the Orion and M17 regions. Density Functional Theory calculations have been carried out on DPAH+ molecules to see the variations in the spectral behaviour from that of a pure PAH. DPAH+ molecules show features that arise due to the stretching of the aliphatic C-D bond. Deuterated PAHs have been previously reported as carriers for such features. However, preferred conditions of ionization of PAHs in the interstellar medium (ISM) indicates the possibility of the formation of DPAH+ molecules. Comparison of band positions of DPAH+ s shows reasonable agreement with the observations. We report the effect of size of the DPAH+ molecules on band positions and intensities. This study also reports a D/H ratio ([D/H]sc ; the ratio of C-D stretch and C-H stretch bands per [D/H]num ) that is decreasing with the increasing size of DPAH+ s. It is noted that large DPAH+ molecules (no. of C atoms ~ 50) match the D/H ratio that has been estimated from observations. This ratio offers prospects to study the deuterium abundance and depletion in the ISM.