In addition to its atomic and molecular forms, interstellar hydrogen also exists in the ionized state. Some of this gas is confined to HII regions surrounding individual O and B stars. Stellar Lyman continuum radiation, produced by hydrogen recombinations in which the photon energy exceeds 13.6 eV, ionizes a spherical volume several parsecs in radius. Recombining electrons and ions in this region emit a plethora of lines, including the optical Balmer series of
Figure 2.4 Emission in 1.3 cm continuum radiation (white contours) from the Orion Nebula. The optical photograph is a negative image in the Ha line.
hydrogen. Transitions between much higher levels of the atom produce radiation at centimeter wavelengths. This emission provides a radio beacon for HII regions over large distances.
Figure 2.4 shows the best studied HII region, that in Orion. Here, radio contours at 1.3 cm are overlaid on a negative Ha image. The contours are centered on the Trapezium stars, which also illuminate much of the region in Ha. Notice how much more symmetric the radio emission appears than the optical line, which is readily attenuated by dust. By summing the flux from such radio peaks over the Galactic plane, the total surface density can be obtained (Figure 2.3). Like the molecular component, the HII gas peaks at a radius near 5 kpc. Since HII regions are created by young, massive stars, this similarity further establishes the association of star formation activity with molecular gas. The total mass contained in these regions is about 1 x 108 Mq, an order of magnitude less than the molecular or atomic hydrogen masses.2
The presence of emission lines from the heavier elements in HII regions allows reconstruction of the chemical composition of interstellar gas. Analysis of the absorption lines from stars lying behind HI clouds is another means to this end. The standard of comparison in such studies is solar composition, i. e., the elemental distribution found in the solar photosphere and in primitive solar system bodies such as meteorites. Table 2.1 lists number abundances relative to hydrogen of the most prevalent elements in gas of solar composition. Also shown are observed abundances in the HII region M42. It is apparent that the metals in this region are systematically depleted relative to the solar standard. Such depletion is generally observed in interstellar gas and can be several orders of magnitude for iron and calcium. Since the total metallicity is presumed to be uniform throughout the solar neighborhood, this trend is evidence that an increasing portion of heavier atoms is locked up in solid matter, i. e., interstellar grains.
2 We stress that this mass estimate includes only discrete HII regions. A larger amount of gas is contained in the warm ionized medium, introduced below.
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