H⁺ region

An H II region is an astrophysical object found in theinterstellar medium and consisting in a cloud of hotplasma of ionized hydrogen and electrons at about 10,000 K. The name stems from the old appellation H II for the ionized state of hydrogen (now H⁺), as opposed to the neutral state H I. The plasma is due to photoionization by the ultraviolet radiation from hot massive stars ofspectral typeO and B. The stars are generally born in a giant molecular cloud and ionize the gas around them that was not condensed into stars. The occasional recombination of a proton and an electron produces a cascade of energy with emission of photons that explains why H II regions appear as luminous reddish nebulae. Their spectrum contains mainly strong H I recombination lines as well as forbidden lines such as those from ionized oxygen and nitrogen ([O III], [O II], [N II]). The plasma is also a source of free–free (bremsstrahlung) emission mainly detected in the radio...

Authors and Affiliations

1. LESIA, Observatoire Paris-Site de Meudon, 5 Pl Jules Janssen, Meudon, 92195, France

   Prof. Daniel Rouan

Corresponding author

Correspondence toDaniel Rouan.

Editors and Affiliations

1. Astrophysicist, Laboratoire d’Astrophysique de Bordeaux, BP 89, 33270, Floirac, France

   Muriel Gargaud

2. Departamento de Planetología y Habitabilidad Centro de Astrobiología (CSIC-INTA), Universidad Autónoma de Madrid Campus Cantoblanco, Torrejón de Ardoz, 28049, Madrid, Spain

   Ricardo Amils

3. Department of Astrophysics, Centro de Astrobiología (INTA-CSIC) Ctra de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain

   José Cernicharo Quintanilla

4. Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington, DC, 20015, USA

   Henderson James (Jim) Cleaves II

5. Department of Astronomy, University of Massachusetts Lederle Graduate Research, 710 North Pleasant Street, Amherst, MA, 01003-9305, USA

   William M. Irvine

6. GEOTOP & Départment des Sciences de la Terre et de l’Atmosphère, Université du Québec à Montréal, CP 8888, succ. Centre-Ville, Montréal, QC, H3C 3P8, Canada

   Daniele L. Pinti

7. Astrobiology, CNES/DSP/EU, 2 place Maurice-Quentin, 75039, Paris, France

   Michel Viso

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