An Etymological Dictionary of Astronomy and Astrophysics
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An → ionization resulting from a → collision.

→ collisional; → ionization.

The ionization of → interstellar medium (ISM) gas by → cosmic rays. Cosmic rays are a primary source of ionization, competing with stellar → ultraviolet photons and → X-rays produced by embedded → young stellar objects. Cosmic rays play a key role in the chemistry and dynamics of the interstellar medium. The ionization fraction in turn drives the chemistry of → molecular clouds and controls the coupling of the gas with the Galactic → magnetic field. Moreover, cosmic rays represent an important source of → heating for → molecular clouds because the energy of primary and secondary electrons produced by the ionization process is in large part converted into heat by → inelastic collisions with ISM atoms and → molecules (see, e.g., Padovanit et al., 2009, arXiv:0904.4149).

→ cosmic; → ray; → ionization.

An → ionization front of → H II regions whose expansion speed is comparable to the → sound speed in the gas (~ 10 km/sec for hydrogen at 10⁴ K). A D-type ionization front results from → R-type ionization front when its propagation speed decreases as the volume of gas ahead of the ionization front grows. If front velocity is equal to a lower limit (C₁² / 2C₂, where C₁ and C₂ are the sound speed ahead and behind the front respectively), the front is called D critical.

D referring to a dense gas; → type; → ionization; → front.

The number of electrons a neutral atom has lost in an ionizing physical process (radiation, shock, collision). In spectroscopy, the degree of ionization is indicated by a Roman numeral following the symbol for the element. A neutral atom is indicated by the Roman numeral I, a singly ionized atom, one which has lost one electron, is indicated by II, and so on. Thus O VI indicates an oxygen atom which has lost five electrons.

→ degree; → ionization.

Chemistry: A process in which all charged species are removed from a solution.
The return of an ionized gas to its non-ionized state after the ionizing source has been removed.

From → de- + → ionization.

→ reionization epoch.

→ epoch; → reionization.

The loss of orbital electrons by an atom of a crystal lattice which has undergone a high-energy collision.

→ impact; → ionization.

The process by which ions are produced, typically occurring by interaction with electromagnetic radiation ("photoionization"), or by collisions with atoms or electrons ("collisional ionization").

Verbal noun of → ionize.

A quantity used in studies of → emission nebulae to convert the → ionic abundance of a given chemical element to its total → elemental abundance. The elemental abundance of an element relative to hydrogen is given by the sum of abundances of all its ions. In practice, not all the ionization stages are observed. One must therefore correct for unobserved stages using ICFs. A common way to do this was to rely on → ionization potential considerations. However, → photoionization models show that such simple relations do not necessarily hold. Hence, ICFs based on grids of photoionization models are more reliable. Nevertheless here also care should be taken for several reasons: the atomic physics is not well known yet, the ionization structure of a nebula depends on the spectral energy distribution of the stellar radiation field, which differs from one model to another, and the density structure of real nebulae is more complicated than that of idealized models (see, e.g., Stasińska, 2002, astro-ph/0207500, and references therein).

→ ionization; → correction; → factor.

Same as → ionization potential.

→ ionization; → energy.

An abrupt discontinuity between an H II region and the molecular cloud in which it has formed. In this transition region interstellar gas changes from a mostly neutral state to a mostly ionized state.

→ ionization; → front.

A ratio representing the number of ionizing photons to the number of electrons in a nebular emitting region.

→ ionization; → parameter.

The energy required to remove an electron from an isolated atom or molecule. The ionization potential for hydrogen is 13.6 eV, which corresponds to an ultraviolet ionizing photon with a wavelength of 912 A. Also called → ionization energy.

→ ionization; → potential.

The spatial distribution of ionic species around an ionization source according to their → ionization potentials. The higher the ionization potential, the nearer to the source the corresponding ions will be.

→ ionization; → stratification.

An H II region whose → exciting star(s) do not have enough → Lyman continuum photons to ionize the whole region. → density-bounded H II region.

→ ionization; → bounded; → H II region.

A spectral line arising from a transition between atomic levels with an ionization potential below approximately 15 electron-volts.

→ low; → ionization; → line.

Same as → LINER.

→ low; → ionization; → nuclear; → emission; → line; → region.

The ionization of air molecules by the heat generated when a meteorite enters the atmosphere.

→ meteorite, → ionization.

One of several zones of the stellar interior where increased → opacity can provide the → kappa mechanism to drive → pulsations. See also → Kramers' law. In these zones where the gases are partially ionized, part of the energy released during a layer's compression can be used for further ionization, rather than raising the temperature of the gas. Partial ionization zones modulate the flow of energy through the layers of the star and are the direct cause of → stellar pulsation. The partial ionization zones were first identified by the Russian astronomer Sergei A. Zhevakin (1916-2001) in the 1950s. In most stars there are two main ionization zones. The hydrogen partial ionization zone where both the ionization of neutral hydrogen (H ↔ H⁺ + e^-) and the first ionization of helium (He ↔ He⁺ + e^-) occurs in layers with a characteristic temperature of 1.5 x 10⁴ K. The second, deeper zone is called the He⁺ partial ionization zone, and involves the second ionization of helium (He⁺↔ He⁺⁺ + e^-), which occurs deeper at a characteristic temperature of 4 x 10⁴ K. The location of these ionization zones within the star determines its pulsational properties. In fact if the → effective temperature of the star is ≥ 7500 K, the pulsation is not active, because the ionization zones will be located very near to the surface. In this region the density is quite low and there is not enough mass available to drive the oscillations. This explains the blue (hot) edge of the instability strip on the → H-R diagram. Otherwise if a star's surface temperature is too low, ≤ 5500 K, the onset of efficient convection in its outer layers may dampen the oscillations. The red (cool) edge of the instability strip is believed to be the result of the damping effect of convection. He⁺ ionization is the driving agent in → Cepheids. See also → gamma mechanism.

→ partial; → ionization; → zone.

The physical process in which an incident high-energy photon ejects one or more electrons from an atom, ion, or molecule.

→ photo- + → ionization.