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Photodissociation region

The image shows the 4 primary zones of a photodissociation region: the molecular zone, the dissociation front, the ionization front, and the fully ionized flow of gas.[1]

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]

  1. ^ "Anatomy of a Photodissociation Region". Webb. Retrieved 2025-02-13.
  2. ^ a b 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. Bibcode:1999RvMP...71..173H. doi:10.1103/RevModPhys.71.173.
  3. ^ "Webb Captures Top of Iconic Horsehead Nebula in Unprecedented Detail - NASA Science". 2024-04-29. Retrieved 2025-02-13.
  4. ^ Wolfire, Mark G.; Vallini, Livia; Chevance, Mélanie (September 2022). "Photodissociation and X-Ray-Dominated Regions". Annual Review of Astronomy and Astrophysics. 60: 247–318. doi:10.1146/annurev-astro-052920-010254. ISSN 0066-4146.
  5. ^ "PhotoDissociation Region Toolbox". dustem.astro.umd.edu. Retrieved 2025-02-13.
  6. ^ Hwang, Jihye; Pattle, Kate; Parsons, Harriet; Go, Mallory; Kim, Jongsoo (2023-03-14), Magnetic fields in the Horsehead Nebula, arXiv, doi:10.48550/arXiv.2303.07628, arXiv:2303.07628, retrieved 2025-02-13
  7. ^ Tielens, A.G.G.M. (1993). "Photodissociation Regions and Planetary Nebulae". Symposium - International Astronomical Union. 155: 155–162. Bibcode:1993IAUS..155..155T. doi:10.1017/S0074180900170330.
  8. ^ Hollenbach, D. J.; Tielens, A. G. G. M. (1997). "Dense photodissociation regions". Annual Review of Astronomy and Astrophysics. 35: 179–215. Bibcode:1997ARA&A..35..179H. doi:10.1146/annurev.astro.35.1.179.
  9. ^ 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. arXiv:2401.06941. Bibcode:2024MNRAS.529.1680E. doi:10.1093/mnras/stae102.
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