Analyzing interstellar infrared spectrum by circumcoronene (C54H18) related molecules

TL;DR

This study investigates interstellar infrared spectra by modeling PAH molecules, especially circumcoronene derivatives, to identify carriers and explain observed spectral variations in different astronomical objects.

Contribution

It extends previous work by analyzing circumcoronene-related molecules, identifying specific PAH cations that reproduce key IR features and explaining spectral types through molecular mixtures.

Findings

Type-B molecules reproduce major IR intensity ratios.

Neutral circumcoronene shows a strong 11.1 μm peak.

Mixtures of different molecular types explain spectral variations.

Abstract

It is very important to identify carrier molecules of astronomical interstellar infrared spectrum (IR) to understand chemical evolution step of polycyclic aromatic hydrocarbon (PAH) in the universe. In our previous study, it was suggested that coronene (C24H12) related PAH could reproduce such IR by the first principles quantum chemical calculation. In this paper, PAH candidates were enhanced to circumcoronene (C54H18) related molecules. Well known PAH oriented wavelength of 6.2, 7.7, 8.6, 11.3, and 12.7 micrometer were reproduced well by a void induced mono-cation (C53H18) having hydrocarbon two pentagons combined with 17 hexagons. Typical astronomical object are the red rectangle nebula and NGC6946, which is categorized as Type-B sectrum. Neutral circumcoronene show very strong IR peak at 11.1 micrometer, which is noted as Type-A spectrum observed in NGC1316 and NGC4589. Ubiquitously well observed spectrum was noted as Type-C, which could be explained by a suitable combination of Type-B and Type-A. Type-C objects are NGC7023, NGC2023 and so many. It should be noted that a dehydrogenated pure carbon mono-cation molecule (C53) show IR peaks at 6.3, 7.8, 8.5 micrometer, but no peak at 11.3 micrometer, which newly defined as Type-E. Such a strange characteristic can contribute to explain IR intensity ratio. Observed intensity ratio between peaks of 6.2, 7.7, 8.6, 11.3, and 12.7 micrometer each other were compared with above calculated IR intensities. Major intensity ratio was well reproduced by Type-B molecules. Variation of observed intensity ratio could be explained by a mixture degree between Type-B and Type-A, and also a mixture degree between Type-B and Type-E.