Astronomy:Photodissociation region
In astrophysics, photodissociation regions (or photon-dominated regions, PDRs) are predominantly neutral regions of the interstellar medium in which far ultraviolet photons strongly influence the gas chemistry and act as the most important source of heat.[2] They constitute a sort of shell around sources of far-UV photons at a distance where the interstellar gas is dense enough, and the flux from the photon source is no longer strong enough, to strip electrons from the neutral constituent atoms.[3] Despite being composed of denser gas, PDRs still have too low a column density to prevent the penetration of far-UV photons from distant, massive stars. PDRs are also composed of a cold molecular zone that has the potential for star formation.[4] They achieve this cooling by far-infrared fine line emissions of neutral oxygen and ionized carbon.[5] It is theorized that PDRs are able to maintain their shape by trapped magnetic fields originating from the far-UV source.[6] A typical and well-studied example is the gas at the boundary of a giant molecular cloud.[2] PDRs are also associated with HII regions, reflection nebulae, active galactic nuclei, and Planetary nebulae.[7] All of a galaxy's atomic gas and most of its molecular gas is found in PDRs.[8]
The closest PDRs to the Sun are IC 59 and IC 63, near the bright Be star Gamma Cassiopeiae.[9]
History
The study of photodissociation regions began from early observations of the star-forming regions Orion A and M17 which showed neutral areas bright in infrared radiation lying outside ionised HII regions.[8]
References
- ↑ "Anatomy of a Photodissociation Region" (in en). https://webbtelescope.org/contents/media/images/2021/024/01F5KKSNNM6YWR7MNMDJ2NEBCN?news=true..
- ↑ 2.0 2.1 Hollenbach, D.J.; Tielens, A.G.G.M. (1999). "Photodissociation regions in the interstellar medium of galaxies". Reviews of Modern Physics 71 (1): 173–230. doi:10.1103/RevModPhys.71.173. Bibcode: 1999RvMP...71..173H. https://zenodo.org/record/1233981.
- ↑ "Webb Captures Top of Iconic Horsehead Nebula in Unprecedented Detail - NASA Science" (in en-US). 2024-04-29. https://science.nasa.gov/missions/webb/webb-captures-top-of-iconic-horsehead-nebula-in-unprecedented-detail/.
- ↑ Wolfire, Mark G.; Vallini, Livia; Chevance, Mélanie (September 2022). "Photodissociation and X-Ray-Dominated Regions" (in en). Annual Review of Astronomy and Astrophysics 60: 247–318. doi:10.1146/annurev-astro-052920-010254. ISSN 0066-4146.
- ↑ "PhotoDissociation Region Toolbox". https://dustem.astro.umd.edu/.
- ↑ Hwang, Jihye; Pattle, Kate; Parsons, Harriet; Go, Mallory; Kim, Jongsoo (2023). "Magnetic Fields in the Horsehead Nebula". The Astronomical Journal 165 (5): 198. doi:10.3847/1538-3881/acc460.
- ↑ Tielens, A.G.G.M. (1993). "Photodissociation Regions and Planetary Nebulae". Symposium - International Astronomical Union 155: 155–162. doi:10.1017/S0074180900170330. Bibcode: 1993IAUS..155..155T.
- ↑ 8.0 8.1 Hollenbach, D. J.; Tielens, A. G. G. M. (1997). "Dense photodissociation regions". Annual Review of Astronomy and Astrophysics 35: 179–215. doi:10.1146/annurev.astro.35.1.179. Bibcode: 1997ARA&A..35..179H. https://zenodo.org/record/1234927.
- ↑ Eiermann, Jacob M. et al. (April 2024). "The 3D geometry of reflection nebulae IC 59 and IC 63 with their illuminating star gamma Cas". Monthly Notices of the Royal Astronomical Society 529 (2): 1680–1687. doi:10.1093/mnras/stae102. Bibcode: 2024MNRAS.529.1680E.
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