Reproduction of Interstellar Infrared Spectrum of Reflection Nebula NGC2023 By A Hydrocarbon Pentagon-Hexagon Combined Molecule

TL;DR

This study demonstrates that ionized hydrocarbon molecules with pentagon-hexagon structures can accurately reproduce the infrared spectrum observed in the reflection nebula NGC2023, providing insights into interstellar PAH carriers.

Contribution

The paper introduces specific hydrocarbon pentagon-hexagon combined molecules as carriers for interstellar infrared spectra, validated by detailed spectral comparison.

Findings

Successful reproduction of observed IR bands by model molecules

Hydrocarbon pentagon-hexagon structures match interstellar spectra

Single molecule models can explain complex IR features

Abstract

Reflection Nebula NGC2023 shows specific interstellar infrared spectrum due to polycyclic aromatic hydrocarbon (PAH) in a wide wavelength range from 5 to 20 micrometer. By our previous quantum chemistry calculation, it was suggested that a molecule group having hydrocarbon pentagon-hexagon combined skeleton could reproduce ubiquitous interstellar infrared spectrum. In this paper, observed NGC2023 spectrum was compared in detail with such carrier candidates. First model molecule was di-cation (C23H12)2+ with two hydrocarbon pentagons combined with five hexagons. Observed strong infrared bands were 6.2, 7.7, 8.6, and 11.2 micrometer. Whereas, calculated strong peaks were 6.4, 7.5, 7.7, 8.5, and 11.2 micrometer. Observed weaker bands from 10 to 15 micrometer were 11.0, 12.0, 12.7, 13.5, and 14.2 micrometer, which were reproduced well by computed bands as 10.9, 12.0, 12.6, 13.6, and 14.0 micrometer. From 15 to 20 micrometer, observed 15.8, 16.4, 17.4, 17.8, and 19.0 micrometer were correlated with calculated 15.6, 16.5, 17.2, 18.2, and 18.8 micrometer. It should be noted that we could successfully reproduce interstellar infrared spectrum by applying a single molecule. Second model molecule was (C12H8)3+ with one pentagon combined with two hexagons. Again, observed strong bands at 6.2, 7.7, 8.6, 11.2 and 12.7 micrometer were successfully computed as 6.3, 7.4, 7.7, 8.6, 11.1, and 12.8 micrometer. It was concluded that by introducing hydrocarbon pentagon-hexagon combined ionized molecules, interstellar PAH oriented infrared spectrum could be successfully reproduced.