Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: Constraints from the Orion Bar as Observed by the James Webb Space Telescope
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arXiv
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2025
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| _version_ | 1866913717814296576 |
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| author | Yang, Xuejuan Li, Aigen |
| author_facet | Yang, Xuejuan Li, Aigen |
| contents | 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 ${N_{\rm D,ali}}/{N_{\rm H}}\approx3.4\%$, 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 $\approx14\%$ on the deuteration degree, which is capable of accounting for an appreciable fraction of the missing D budget. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2503_02204 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: Constraints from the Orion Bar as Observed by the James Webb Space Telescope Yang, Xuejuan Li, Aigen Astrophysics of Galaxies 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 ${N_{\rm D,ali}}/{N_{\rm H}}\approx3.4\%$, 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 $\approx14\%$ on the deuteration degree, which is capable of accounting for an appreciable fraction of the missing D budget. |
| title | Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: Constraints from the Orion Bar as Observed by the James Webb Space Telescope |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2503.02204 |