Chemical composition of H II regions

The chemical composition of H II regions (the numbers of atoms of each chemical element, relative to hydrogen) can be estimated from nebular spectra. Each element is found in more than one stage of ionization, so the first step is to use the emission-line strengths of each stage of ionization, relative to those of the hydrogen lines, to obtain the abundance of that particular stage of ionization. All abundant elements have some stages of ionization that produce observable emission lines. On the other hand, some elements, such as argon, sulfur, and carbon, have important ions that do not show easily observable lines. Elaborate computer calculations predict the ionization structure of gas ionized by a hot star whose temperature is determined by its spectrum. The calculations then provide predictions of the abundances of the invisible ions, relative to the observable, and the total elemental abundance follows.

The main difficulty with this straightforward procedure is that there are two methods for determining the observed ionic abundances; each should be reliable, but they give quite different answers. By far the easier method of determining ionic abundances is to observe the bright lines produced by collisions between the ion and energetic electrons. The brightest lines from this process in the entire spectrum of H II regions arise from either O⁺ or O⁺². These bright lines (and others such as N⁺) are the basis of the abundance determinations in other galaxies.

Alternatively, the ionic abundances can be determined from the very faint emission lines that follow recombination, the process by which the higher stage of ionization captures an electron (usually at low energies) into a high level of the ion. Following recombination, there is a cascade from the high energy levels to the ground state, with photons in the observed emission line being emitted at each downward transition. These emission lines are fainter than the hydrogen emissions by roughly the ratio of abundances, which is more than 1,000, so only in bright nebulae can this method be used. However, modern spectrographs on large telescopes have provided strengths of these faint recombination lines for many objects, with good agreement among observers. Checks are provided by comparison of several lines of the same stage of ionization. The relative strengths of the observed lines agree well with the expectations of the cascading process in the excited ions.

The results are interesting and controversial. For H II regions that are bright enough for the faint heavy-element recombination lines to be measured, the carbon, nitrogen, neon, and oxygen abundances from recombination lines are uniformly about 1.8 times those from the collisionally excited lines. A common interpretation is that there are strong temperature fluctuations within the nebulae. In the warmer regions the collisionally excited lines are strongly overproduced per heavy ion, so fewer heavy ions are needed to account for the observed line strengths. The hydrogen lines are hardly affected by the postulated temperature fluctuations. The temperature fluctuations, which must be large (about 20 percent of the average), are unexplained. Turbulence and magnetic fields are prime suspects.

The nebular temperature can be estimated directly from collisional lines alone by comparing emission lines from high-energy levels, populated by collisions, with lines from lower levels. This process can be carried out at various places on the sky within each nebula, and large-scale temperature fluctuations would appear as variations from place to place. However, no such variations have been convincingly detected. Some astronomers have proposed that there are chemical inhomogeneities within nebulae that give rise to the differences between the abundances derived from recombination and collisionally excited lines, but how such variations could be maintained is unexplained.

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Nevertheless, abundances are estimated on the basis of a simple interpretation of the mysterious postulated temperature excursions, but no better procedure has been suggested to deal with the startling discrepancies in the derived abundances. The other class of ionized nebulae, the planetary nebulae, show the same effect. The local estimated abundances (say, for the Orion Nebula) are roughly solar. The abundances of heavy elements per million hydrogen atoms are 500 for carbon, 80 for nitrogen, 600 for oxygen, and 100 for neon. There is a gradient of these abundances within the Milky Way Galaxy. At a distance halfway to the centre, 12,000 light-years inward, they are 50 percent larger than locally. Beyond Earth the gradient seems to persist, but there are very few observations. Helium is about 0.1 times as abundant as hydrogen, by number of atoms, throughout the Milky Way Galaxy.

Except for a few cases, compositions of nebulae in galaxies outside the Milky Way Galaxy are measured by collisionally excited lines. The Large Magellanic Cloud has compositions that are uniformly about one-half those of the local Milky Way for oxygen, neon, argon, and sulfur and are one-quarter for carbon and nitrogen. It appears that the first group of elements must be manufactured together, presumably in massive stars, and ejected together into the interstellar gas. Stars of a different (probably lower) mass must produce carbon and nitrogen. Planetary nebulae suggest the same scenario.

The abundance of helium in nebulae has received considerable attention because the helium content of the oldest objects provides clues to the origin of the universe. The value cited above for the Orion Nebula is in agreement with the predictions of the big-bang model.