photodissociation region

{{short description|Gaseous parts of the interstellar medium which are heated by UV photons}}

File:STScI-01F686BHJ60D8GR8VVRMA21V8J.png

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.{{cite journal | last1=Hollenbach | first1=D.J. | last2=Tielens | first2=A.G.G.M. | journal=Reviews of Modern Physics | date=1999 | title=Photodissociation regions in the interstellar medium of galaxies | volume=71 | issue=1 | pages=173–230 | bibcode=1999RvMP...71..173H | doi=10.1103/RevModPhys.71.173 | url=https://zenodo.org/record/1233981 }} 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.{{Cite web |date=2024-04-29 |title=Webb Captures Top of Iconic Horsehead Nebula in Unprecedented Detail - NASA Science |url=https://science.nasa.gov/missions/webb/webb-captures-top-of-iconic-horsehead-nebula-in-unprecedented-detail/ |access-date=2025-02-13 |language=en-US}} 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.{{Cite journal |last1=Wolfire |first1=Mark G. |last2=Vallini |first2=Livia |last3=Chevance |first3=Mélanie |date=September 2022 |title=Photodissociation and X-Ray-Dominated Regions |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=60 |pages=247–318 |doi=10.1146/annurev-astro-052920-010254 |doi-access=free|issn=0066-4146|arxiv=2202.05867 }} They achieve this cooling by far-infrared fine line emissions of neutral oxygen and ionized carbon.{{Cite web |title=PhotoDissociation Region Toolbox |url=https://dustem.astro.umd.edu/ |access-date=2025-02-13 |website=dustem.astro.umd.edu}} It is theorized that PDRs are able to maintain their shape by trapped magnetic fields originating from the far-UV source.{{cite journal |arxiv=2303.07628 |doi=10.3847/1538-3881/acc460 |doi-access=free |title=Magnetic Fields in the Horsehead Nebula |date=2023 |last1=Hwang |first1=Jihye |last2=Pattle |first2=Kate |last3=Parsons |first3=Harriet |last4=Go |first4=Mallory |last5=Kim |first5=Jongsoo |journal=The Astronomical Journal |volume=165 |issue=5 |page=198 }} A typical and well-studied example is the gas at the boundary of a giant molecular cloud. PDRs are also associated with HII regions, reflection nebulae, active galactic nuclei, and Planetary nebulae.{{cite journal | last1=Tielens | first1=A.G.G.M. | title=Photodissociation Regions and Planetary Nebulae | journal=Symposium - International Astronomical Union | date=1993 | volume=155 | pages=155–162 | doi=10.1017/S0074180900170330 | bibcode=1993IAUS..155..155T | doi-access=free}} All of a galaxy's atomic gas and most of its molecular gas is found in PDRs.{{cite journal | first1=D. J. | last1=Hollenbach | first2=A. G. G. M. | last2=Tielens | title=Dense photodissociation regions | journal=Annual Review of Astronomy and Astrophysics | date=1997 | volume=35 | pages=179–215 | bibcode=1997ARA&A..35..179H | doi=10.1146/annurev.astro.35.1.179 | url=https://zenodo.org/record/1234927 }}

The closest PDRs to the Sun are IC 59 and IC 63, near the bright Be star Gamma Cassiopeiae.{{cite journal | title=The 3D geometry of reflection nebulae IC 59 and IC 63 with their illuminating star gamma Cas | last1=Eiermann | first1=Jacob M. | last2=Caputo | first2=Miranda | last3=Lai | first3=Thomas S. -Y. | last4=Witt | first4=Adolf N. | journal=Monthly Notices of the Royal Astronomical Society | volume=529 | issue=2 | pages=1680–1687 | date=April 2024 | doi=10.1093/mnras/stae102 | doi-access=free | arxiv=2401.06941 | bibcode=2024MNRAS.529.1680E | display-authors=1 }}