2.1. Spectroscopy of H II Regions and Planetary Nebulae
Most of the information we have on abundances in spiral and irregular galaxies have come from spectroscopy of H II regions. This is logical since H II regions are luminous and have high surface brightness (in the emission lines) compared to individual stars in galaxies. One can think of an H II region as an efficient machine for converting the extreme ultraviolet radiation of a hot, massive star into a few narrow emission lines, leading to a very luminous object in the optical/IR bands. As a result, observations of H II regions have typically provided our first look at abundances within galaxies. Indeed, emission lines are now being used to probe the ISM of galaxies at redshifts greater than 2, as will be discussed by Pettini in these proceedings.
Elements that are readily observed in the visible spectrum of H II regions include O, N, Ne, S, and Ar. With the exception of O, all of these elements may have important ionization states that emit only in the ultraviolet or infrared (for example, Ne+, N+2, S+3). If we add UV spectroscopy we can study C and Si. Figure 1 shows an HST spectrum of one H II region in the SMC, showing the rich variety of forbidden emission lines and H, He recombination lines in the UV and optical spectrum. Other abundant heavy elements (such as Fe or Mg) may be observed in photoionized nebulae. One must always keep in mind that many elements in the ISM are strongly depleted onto grains, which affects the total abundance. This is an important but poorly known factor in many cases (such as O and C). Another caution is that H II regions show the composition of the present-day ISM, and are insensitive to the evolution of abundances with time.
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Figure 1. Hubble Space Telescope UV/optical spectrum of the H II region N88A in the Small Magellanic Cloud. |
Planetary nebulae (PNs) are essentially H II regions created by the ionization of a red giant envelope by the exposed hot stellar core, so spectroscopy of a PN provides information on a similar variety of elements as H II regions, with the same caveats. There are a number of significant differences, however. PNs are much less luminous than H II regions, so observations suitable for abundance measurements are restricted to the nearest galaxies; at the present time, measuring abundances in PNs is challenging in galaxies as close as M31 (Jacoby & Ciardullo 1999). Another difference is that the PN abundances are altered from the original stellar composition by nucleosynthesis - He, C, and N are often enriched, and even O may be affected. Thus, the use of PNs to measure abundances across galaxies must be pursued with caution. Nevertheless, measurements of PNs offer a means of measuring abundances across galaxies and their evolution over the range of ages of PN progenitors (a few tens to a few thousands of Myr). Stasinska will discuss PN abundance measurements in her lectures.
The observational and analytic techniques for determining abundances in H II regions have been discussed at great length by Skillman (1998) at the VIIIth Canary Island Winter School and by Stasinska in her lectures here. I will add a few remarks here on observing and deriving abundances.