Categorize Interstellar Infrared Spectrum by Polycyclic Pure-Carbon-Molecule and Hydrocarbon-Molecule

TL;DR

This study classifies interstellar infrared spectra into three types using quantum chemical calculations, identifying specific molecules responsible for each type and suggesting mixtures explain observed spectral variations.

Contribution

The paper introduces a new categorization of interstellar IR spectra based on quantum calculations, linking them to specific pure-carbon and hydrocarbon molecules, and proposes a mixture model for spectral diversity.

Findings

Type-A spectra linked to C23 dication with characteristic peaks.

Type-B spectra best matched with modified PAH molecules.

Type-C spectra characterized by a strong 11.3 micrometer peak.

Abstract

By applying quantum chemical calculation, interstellar infrared spectrum was categorized to three classes. Type-A show unusual feature of strong peaks at 11.3,12.9, and 14.0 micrometer. Usually observed 6.2, 7.7, and 8.6 micrometer bands are weak or not recognized. Typical examples are NGC1316 and NGC4589. Such spectrum could be identified for the first time by pure carbon molecule (C23)2+ (dication) having two carbon pentagons combined with five hexagons. Calculated spectrum coincided well at 11.3, 13.0, and 14.0 micrometer. Also we could find more coincidence at 5.2, 5.6, 7.6, 8.8,10.6,15.7 and 17.2 micrometer. Type-B is ubiquitously observed IR, but show medium strength at 11.3 micrometer. Examples are NGC6946 and the red triangle nebula. Coronene modified PAH (C23H12)2+ show best coincidence for both wavelength and strength. Type-C is usualy observed one featuring very strong peak at 11.3 micrometer. Examples are NGC7023, NGC2023 and M17SW. One capable expanation of large 11.3 micrometer band is a mixture of Type-A and Type-B. Combination of polycyclic pure-carbon-molecule and hydrocarbon-molecule may give a variety of IR spectrum.