The Carriers of the Interstellar Unidentified Infrared Emission Features: Aromatic or Aliphatic?

TL;DR

This paper investigates the chemical structure of interstellar carriers responsible for unidentified infrared emission features, concluding they are mainly aromatic with minimal aliphatic content, supporting the PAH model.

Contribution

It provides observational evidence that UIE carriers are predominantly aromatic, challenging the hypothesis of mixed aromatic-aliphatic organic nanoparticles.

Findings

Aliphatic fraction of UIE carriers is less than 15%.

UIE features are mainly due to aromatic PAH molecules.

PAH model aligns with astronomical observations.

Abstract

The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer, commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules, have been recently ascribed to coal- or kerogen-like organic nanoparticles with a mixed aromatic-aliphatic structure. However, we show in this Letter that this hypothesis is inconsistent with observations. We estimate the aliphatic fraction of the UIE carriers based on the observed intensities of the 3.4 and 6.85 micrometer emission features by attributing them exclusively to aliphatic C-H stretch and aliphatic C-H deformation vibrational modes, respectively. We derive the fraction of carbon atoms in aliphatic form to be <15%. We conclude that the UIE emitters are predominantly aromatic with aliphatic material at most a minor part of the UIE carriers. The PAH model is consistent with astronomical observations and PAHs dominate the strong UIE bands.