HII

HII
Name	HII
Othernames	Ionized hydrogen, protonic hydrogen
Caption	Schematic of ionized hydrogen in an H II region
Formula	H+
Appearance	Plasma
Phase	Ionized gas
Discovered	20th century spectroscopy
Notable	Dominant component of H II regions

HII

HII denotes ionized hydrogen, the state in which a hydrogen atom has lost its electron and exists as a free proton. It is a fundamental constituent of astrophysical plasmas and plays a central role in the interstellar medium around young massive stars, in emission nebulae, and in the ionized components of galaxies. Studies of HII connect observational campaigns by the Hubble Space Telescope, theoretical work by investigators at institutions such as the Max Planck Society and Harvard–Smithsonian Center for Astrophysics, and surveys like the Sloan Digital Sky Survey.

Overview

HII refers to hydrogen in its ionized state (H+), distinct from neutral hydrogen (HI) and molecular hydrogen (H2). The identification of ionized hydrogen relied on spectroscopy developed by figures including Joseph von Fraunhofer, William Huggins, and later analyses in the era of Edwin Hubble and Fritz Zwicky. HII dominates the composition of classical emission nebulae such as the Orion Nebula, the Rosette Nebula, and the Carina Nebula, which were catalogued in projects like the Henry Draper Catalogue and mapped by missions including the Spitzer Space Telescope and the Chandra X-ray Observatory. In cosmological contexts, HII plays a role in epochs studied by teams involved with the Planck (spacecraft) mission and the James Webb Space Telescope.

Formation and Properties

Ionization of hydrogen is driven primarily by photoionization from ultraviolet photons emitted by hot stars, particularly O-type and B-type stars such as those found in clusters like the Trapezium Cluster and associations like Cygnus OB2. Collisional ionization in shocks associated with supernova remnants—e.g., Cassiopeia A—and energetic processes near active galactic nuclei such as Messier 87 can also produce HII. Recombination of free electrons with protons yields emission lines described by recombination theory developed by researchers at institutions including the Royal Observatory, Greenwich and the California Institute of Technology. Characteristic emission includes the Balmer series (notably Hα), and forbidden lines from ions such as [O III] and [S II], which were used historically by astronomers like Vesto Slipher and Gustaf Strömberg to infer physical conditions. Electron temperature, electron density, and ionization parameter are measurable properties studied in works from the European Southern Observatory and the National Radio Astronomy Observatory.

H II Regions in Star Formation

HII regions are often co-located with sites of recent star formation, including giant molecular clouds catalogued by surveys from the Atacama Large Millimeter/submillimeter Array and the James Clerk Maxwell Telescope. Massive star formation in complexes like Westerlund 2, NGC 3603, and the Tarantula Nebula produces intense ultraviolet flux that carves ionized bubbles, a process analyzed in models by groups at the Kavli Institute for Theoretical Physics and the Institute of Astronomy, Cambridge. HII regions regulate stellar feedback alongside winds from stars such as Eta Carinae and radiation pressure examined in theoretical frameworks advanced by researchers at Princeton University and Stanford University. Triggered star formation at ionization fronts appears in objects like Bok globules and pillars exemplified by structures in the Eagle Nebula, which were famously imaged by the Hubble Space Telescope.

Observational Characteristics

Ionized hydrogen emits strong optical emission lines, with Hα being prominent in spectra obtained by instruments on the Very Large Telescope, Keck Observatory, and amateur setups following the legacy of observers like Percival Lowell. Radio recombination lines and free–free continuum emission are accessible to observatories such as the Very Large Array and the Parkes Observatory, while infrared fine-structure lines are studied with the Infrared Space Observatory and the Spitzer Space Telescope. Spectroscopic diagnostics developed by astronomers including Donald Osterbrock and Gary Steigman allow determination of metallicity, ionization state, and extinction using comparisons with data from the Sloan Digital Sky Survey and surveys by the European Space Agency. High-resolution imaging from the Hubble Space Telescope and integral-field spectroscopy from instruments like MUSE enable mapping of kinematics, turbulence, and shock fronts in celebrated regions such as Orion Nebula and 30 Doradus.

Role in Galaxy Evolution

Ionized hydrogen regions trace recent star formation and thus serve as indicators in studies by collaborations such as the Sloan Digital Sky Survey and the Galaxy Evolution Explorer. Emission-line diagnostics contribute to classifications using schemes like the Baldwin–Phillips–Terlevich diagram and inform measurements of star-formation rate calibrations used by teams at the Space Telescope Science Institute and the Carnegie Institution for Science. HII regions influence chemical enrichment through feedback processes related to supernova activity in systems ranging from dwarf galaxies like NGC 1569 to spirals like Andromeda Galaxy. On cosmological scales, reionization studies by consortia analyzing data from Planck (spacecraft), Hubble Space Telescope, and James Webb Space Telescope consider the contribution of ionizing photons from early galaxies and HII bubbles during the Epoch of Reionization.

Related Phenomena

Related ionized structures include planetary nebulae such as Ring Nebula and Cat's Eye Nebula, where ionization arises from evolved stars like Sirius B-type remnants. Diffuse ionized gas in the halos of galaxies, sometimes called the warm ionized medium, is mapped in the Milky Way by surveys linked to the Wisconsin H-Alpha Mapper and compared with extraplanar emission in systems like NGC 891. Shock-ionized regions associated with jets from objects such as Herbig–Haro objects and active nuclei in galaxies like Centaurus A show emission-line ratios distinct from photoionized HII regions, a distinction exploited in studies by the Royal Astronomical Society and university research groups worldwide.

Category:Interstellar medium