2025/03/04 by Yang, Xuejuan, Li, Aigen
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences
paper · doi:10.48550/arxiv.2503.02204
The gas-phase abundances of deuterium (D) in the local interstellar medium (ISM) exhibit considerable regional variations. Particularly, in some regions the gas-phase D abundances are substantially lower than the primordial D abundance generated in the Big Bang, after subtracting the astration reduction caused by the Galactic chemical evolution. Deuterated polycyclic aromatic hydrocarbon (PAH) molecules have been suggested as a potential reservoir of the D atoms missing from the gas-phase. Recent observations from the James Webb Space Telescope's Near Infrared Spectrograph have revealed the widespread of deuterated PAHs in the Orion Bar through their aliphatic C--D emission at 4.65 \rm μm and possibly aromatic C--D emission at 4.4 \rm μm as well. To examine the viability of deuterated PAHs as the D reservoir, we model the infrared (IR) emission spectra of small PAH molecules containing various aromatic and aliphatic D atoms in the Orion Bar. We find that small deuterated PAHs exhibit a noticeable emission band at 4.4 or 4.65 \rm μm even if they contain only one aromatic or aliphatic D atom. We derive N\rm D,ali/N\rm H≈3.4%, the deuteration degree of PAHs measured as the number of aliphatic D atoms (relative to H), from the observed intensity ratios of the 4.65 \rm μm band to the 3.3 \rm μm aromatic C--H band. The deuteration degree for aromatically-deuterated PAHs is less certain as C--N stretch also contributes to the observed emission around 4.4 \rm μm. If we attribute it exclusively to aromatic C--D, we derive an upper limit of ≈14% on the deuteration degree, which is capable of accounting for an appreciable fraction of the missing D budget.