Infrared Emission of Specific Polycyclic Aromatic Hydrocarbon Molecules: Indene

By Keith Cowing
Status Report
March 12, 2024
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Infrared Emission of Specific Polycyclic Aromatic Hydrocarbon Molecules: Indene
Top left panel (a): Comparison of the UV absorption of indene with an elevated value at λ < 0.19 µm (“higher Cabs”; black long-dashed line) and that with the absorption at λ < 0.19 µm extrapolated from that at λ > 0.19 µm (“lower Cabs”; magenta dashed line). The UV absorption at λ > 0.19 µm for both cases is the same as Figure 1. Bottom left panel (b): Comparison of the MMP83 interstellar radiation field (red) with the radiation fields represented by different stellar effective temperatures (Teff = 40, 000 K: blue; Teff = 22, 000 K: cyan; and Teff = 8, 000 K: green). All the radiation intensities are normalized at λ = 0.2 µm. Right panel (c): Comparison of the model IR emission spectrum of indene in the diffuse ISM calculated with an elevated UV absorption at λ < 0.19 µm (dashed blue line) and that with the extrapolated UV absorption at λ < 0.19 µm (solid red line). -- astro-ph.GA

Polycyclic aromatic hydrocarbon (PAH) molecules have long been suggested to be present in the interstellar medium (ISM). Nevertheless, despite their expected ubiquity and sustained searching efforts, identifying specific interstellar PAH molecules from their infrared (IR) spectroscopy has so far been unsuccessful.

However, due to its unprecedented sensitivity, the advent of the James Webb Space Telescope (JWST) may change this. Meanwhile, recent years have witnessed breakthroughs in detecting specific PAH molecules (e.g., indene, cyanoindene, and cyanonaphthalene) through their rotational lines in the radio frequencies.

As JWST holds great promise for identifying specific PAH molecules in the ISM based on their vibrational spectra in the IR, in this work we model the vibrational excitation of indene, a molecule composed of a six-membered benzene ring fused with a five-membered cyclopentene ring, and calculate its IR emission spectra for a number of representative astrophysical regions. This will facilitate JWST to search for and identify indene in space through its vibrational bands and to quantitatively determine or place an upper limit on its abundance.

Kaijun Li, Aigen Li, Xuejuan Yang, Taotao Fang

Comments: 8 pages, 6 figures; accepted for publication in MNRAS. arXiv admin note: text overlap with arXiv:2403.05043
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2403.05603 [astro-ph.GA] (or arXiv:2403.05603v1 [astro-ph.GA] for this version)
Submission history
From: Kaijun Li
[v1] Fri, 8 Mar 2024 04:47:09 UTC (374 KB)
Astrobiology, Astrochemistry,

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻