Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: Constraints from the Orion Bar as Observed by the James Webb Space Telescope
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 ca...
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2025-01-01
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| author | X. J. Yang Aigen Li |
| author_facet | X. J. Yang Aigen Li |
| author_sort | X. J. Yang |
| collection | DOAJ |
| description | 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 μ m and possibly aromatic C–D emission at 4.4 μ m 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 μ m even if they contain only one aromatic or aliphatic D atom. We derive N _D,ali / N _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 μ m band to the 3.3 μ m 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 μ m. 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. |
| format | Article |
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| publishDate | 2025-01-01 |
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| spelling | doaj-art-33d0f967fe1142e4a6a03661e000bae42025-08-20T02:16:10ZengIOP PublishingThe Astrophysical Journal1538-43572025-01-01983213610.3847/1538-4357/adbd11Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: Constraints from the Orion Bar as Observed by the James Webb Space TelescopeX. J. Yang0https://orcid.org/0000-0002-6605-6512Aigen Li1https://orcid.org/0000-0002-1119-642XDepartment of Physics, Xiangtan University , 411105 Xiangtan, Hunan Province, People’s Republic of China ; xjyang@xtu.edu.cn; Department of Physics and Astronomy, University of Missouri , Columbia, MO 65211, USA ; lia@missouri.eduDepartment of Physics and Astronomy, University of Missouri , Columbia, MO 65211, USA ; lia@missouri.eduThe 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 μ m and possibly aromatic C–D emission at 4.4 μ m 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 μ m even if they contain only one aromatic or aliphatic D atom. We derive N _D,ali / N _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 μ m band to the 3.3 μ m 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 μ m. 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.https://doi.org/10.3847/1538-4357/adbd11Line intensitiesPolycyclic aromatic hydrocarbonsInterstellar line emissionInterstellar moleculesPhotodissociation regions |
| spellingShingle | X. J. Yang Aigen Li Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: Constraints from the Orion Bar as Observed by the James Webb Space Telescope The Astrophysical Journal Line intensities Polycyclic aromatic hydrocarbons Interstellar line emission Interstellar molecules Photodissociation regions |
| title | Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: Constraints from the Orion Bar as Observed by the James Webb Space Telescope |
| title_full | Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: Constraints from the Orion Bar as Observed by the James Webb Space Telescope |
| title_fullStr | Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: Constraints from the Orion Bar as Observed by the James Webb Space Telescope |
| title_full_unstemmed | Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: Constraints from the Orion Bar as Observed by the James Webb Space Telescope |
| title_short | Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: Constraints from the Orion Bar as Observed by the James Webb Space Telescope |
| title_sort | deuterated polycyclic aromatic hydrocarbons in the interstellar medium constraints from the orion bar as observed by the james webb space telescope |
| topic | Line intensities Polycyclic aromatic hydrocarbons Interstellar line emission Interstellar molecules Photodissociation regions |
| url | https://doi.org/10.3847/1538-4357/adbd11 |
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