Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: Constraints from the Orion Bar as Observed by the James Webb Space Telescope

Abstract

The gas-phase abundances of deuterium (D) in the local interstellar medium 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 how widespread deuterated PAHs are in the Orion Bar through their aliphatic C–D emission at 4.65 μm and possibly aromatic C–D emission at 4.4 μm as well. To examine the viability of deuterated PAHs as the D reservoir, we model the infrared 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 μm even if they contain only one aromatic or aliphatic D atom. We derive ND,ali/NH ≈ 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 μm band to the 3.3 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 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.