2.2. Emission Line Abundances for Beginners
In this short overview, I would like to follow along the lines of the review article by Dinerstein (1990). She has divided the abundance analysis of HII region spectra into three different approaches: (1) the "direct" method; (2) the bright-line method; and (3) the photoionization modeling method. I will attempt to emphasize brevity here, as the goal of this section is to act as an entry point, not to be comprehensive overview.
Before beginning a description of the different abundance analysis methods, it is useful to make two introductory remarks. First, I remind the reader that the electron temperature in an HII region reflects the balance between heating and cooling processes. The heating is due to absorption of the ionizing radiation from the exciting star(s), while the cooling is primarily due to collisional excitation of bound electrons (and subsequent radiation) in the ions of heavy elements. Clearly, the electron temperature will have a metallicity dependence: At higher metallicity, more coolants are available, and the electron temperature will will be lower.
Figure 2. A Grotrian diagram of the energy levels in O++. Note that the scale on the left has been expanded by a factor of 100 on the right to show the fine structure lines. For a detailed discussion, see Dinerstein, Lester, & Warner (1985).
Second, lets look at a Grotrian diagram and get a feel for the transition energies involved. Figure 2 shows the Grotrian diagram for O++. The transition energies are plotted in eV, which is convenient when remembering that kT is almost 1 eV (0.86) at 104K which is typical for HII regions. The left hand side of the diagram shows the transitions that are found at optical wavelengths. The strengths of these emission lines are strongly temperature dependent, and thus strongest in the high temperature (low metallicity) HII regions. The right hand side of the diagram shows the fine structure lines. Note that the scale has changed by a factor of 100. The strengths of these fine structure lines will be temperature independent, but linearly proportional to the abundance of O++.