Aliphatics And Aromatics In The Universe: The Pre-JWST Era

By Keith Cowing
Status Report
August 15, 2023
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Aliphatics And Aromatics In The Universe: The Pre-JWST Era
PAH aliphatic fraction (ηali) vs. stellar effective temperature (Teff). PPN: protoplanetary nebula; PN: planetary nebula; RN: reflection nebula; MC: molecular cloud; PDR: photodissociated region; YSO: young stellar object. — astro-ph.GA

The so-called “unidentified infrared emission” (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micron ubiquitously seen in a wide variety of astrophysical regions are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules.

Astronomical PAHs often have an aliphatic component (e.g., aliphatic sidegroups like methyl –CH3 may be attached as functional groups to PAHs) as revealed by the detection in many UIE sources of the aliphatic C–H stretching feature at 3.4 micron. With its unprecedented sensitivity, unprecedented spatial resolution and high spectral resolution, the James Webb Space Telescope (JWST) holds great promise for evolutionizing the studies of aliphatics and aromatics in the universe.

To facilitate analyzing JWST observations, we present a theoretical framework for determining the aliphatic fractions (\eta_ali) of PAHs, the fractions of C atoms in aliphatic units, from the emission intensity ratios of the 3.4 micron feature to the 3.3 micron feature. To demonstrate the effectiveness of this framework, we compile the 3.3 and 3.4 micron UIE data obtained in the pre-JWST era for an as complete as possible sample, and then apply the framework to these pre-JWST data.

We derive a median aliphatic fraction of <\eta_ali>=5.4\% and find that the aliphatic fractions are the highest in protoplanetary nebulae illuminated by cool stars lacking ultraviolet radiation. Nevertheless, the “hardness” of stellar photons is not the only factor affecting the PAH aliphaticity, other factors such as the starlight intensity may also play an important role.

X.J. Yang, Aigen Li

Comments: 30 pages, 15 figures, accepted for publication by ApJS
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2308.04004 [astro-ph.GA] (or arXiv:2308.04004v1 [astro-ph.GA] for this version)
Submission history
From: Xuejuan Yang
[v1] Tue, 8 Aug 2023 02:47:40 UTC (965 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) 🖖🏻