2.2. Spectroscopy of Individual Stars
Stellar spectroscopy has been a very valuable tool for studying the composition and evolution of stars in our Galaxy. Recent improvements in instrumentation and the construction of 8-10m telescopes has allowed this kind of work to be extended to other galaxies. It is not possible yet to do routine spectroscopy of F and G main sequence stars outside the Milky Way, so these studies have concentrated on A and B type supergiants or red giants. Nevertheless, detailed abundance studies of individual stars is not likely to extend far beyond the Local Group for some time because of telescope size limitations.
The supergiants are an important complement to spectroscopy of H II regions, since they sample similar spatial and temporal distributions. Furthermore, they overlap in many of the elements that can be studied: C, N, O, Ne, and so on. On the one hand, the supergiants provide information on elements such as Si, Fe and s-process elements that are depleted into grains in the ISM. On the other hand, the H II regions provide a valuable comparison for He, C, and N which may be affected by internal mixing and nucleosynthesis in the massive stars, which is covered by Norbert Langer's contribution. This is one of the important uncertainties in abundance studies for these stars; others include the degree to which conditions depart from LTE, and the effects of spherical geometry and stellar winds.
Spectroscopy of red giants is well established from Milky Way studies, as discussed by David Lambert. Red giants are valuable because they sample abundances over long time spans, from a 100 Myr to greater than 10 Gyr. A wide variety of elements can be studied, including capture elements, Fe-peak elements, and neutron-capture elements. C, N, and O (and s-process elements in AGB stars) can be affected by internal mixing. Since red giants are fainter than H II regions or supergiants, detailed spectroscopy is limited to the Milky Way's satellite galaxies at present.
For metallicity distributions, one can examine lower spectral resolution diagnostics. The most useful of these has been the Ca II triplet indicator (Armandroff & Da Costa 1991), which uses the combined equivalent width of the Ca II triplet near 8500 Å, calibrated with metallicities of globular clusters, to infer the metallicity [Fe/H] (where the brackets denote the logarithmic abundance relative to that in the Sun). The main uncertainty of this method is that the Ca/Fe abundance ratio can vary depending on the star formation history, so the globular clusters may not provide the correct metallicity calibration for galaxies with a variety of star formation histories. Work needs to be done to calibrate the Ca II triplet with [Ca/H] rather than [Fe/H] to remove this ambiguity.