|Annu. Rev. Astron. Astrophys. 1981. 19:
Copyright © 1981 by . All rights reserved
Like ancient Gaul, abundance data relevant to the study of nucleosynthesis in the course of stellar and galactic evolution can be broadly divided into three parts: abundances of the various elements and their isotopes in the Solar System; composition peculiarities in the visible layers of individual stars, planetary nebulae, young supernova remnants, cosmic ray sources, etc. due to thermonuclear (or other) processes that have taken place in the course of the internal evolution of the stars themselves; and the composition of the interstellar medium (ISM) at different places and times as judged either from direct observations of the ISM at the present time or from characteristics of the various stellar populations that are taken as being identical to those of the ISM at the time and place where the respective stars were born.
The Solar System provides the richest source of data that we have, thereby giving us both a cross section through the effects of past nucleosynthesis in the neighborhood of the Sun and a standard to which the more limited data for other objects can be compared; while abundances in evolved stars, planetary nebulae, and young supernova remnants (SNR) provide direct information on nuclear and mixing processes in stellar evolution that are believed to be responsible for continuing enrichment of the ISM in heavy elements and to be much like the corresponding processes that have produced the abundance distribution in the ISM today, which - in our own neighborhood - is fairly similar to the standard solar-system distribution.
The present article is concerned with the third aspect of astrophysical abundances, relating to the ISM and stellar populations. This aspect, like the others, has benefited from substantial advances in instrumental technique and theoretical modeling in recent years, which have made it possible to make a number of definite assertions about the composition of individual stars and HII regions, and about certain isotopic ratios in the diffuse ISM. The accuracy still varies widely, ranging from perhaps ± 10 percent for the He/H ratio in some HII regions to ± 0.1 to 0.3 dex for typical spectroscopic determinations of specific elements in individual objects. The study of stellar populations in remote galaxies, where only the integrated light can be observed, still presents severe difficulties, and even when HII regions are present and can be fairly accurately analyzed to give gas-phase abundances in the ISM, one cannot automatically assume that the results apply to the underlying stellar population, which in general will cover a wide range of ages.
Owing to limited space, we cannot review here all topics related to our subject; in particular, we can refer only in passing to models of galactic chemical evolution for which the reader should see the articles by Audouze & Tinsley (1976), Tinsley (1980), and Pagel (1979b, 1981). The entire field of astronomical abundances, as it looked six years ago, has been reviewed by Trimble (1975). Other relevant articles, which we shall essentially take as read, are those of van den Bergh (1975: Stellar Populations in Galaxies), Peimbert (1975: Extragalactic Gaseous Nebulae), Baschek (1979: Abundances in Evolved Stars), Kraft (1979: On the Nonhomogeneity of Metal Abundances in Globular Clusters and the Satellite Subsystems of the Galaxy), and Harris & Racine (1979: Globular Clusters in Galaxies). Globular clusters are also reviewed by K.C. Freeman and J. Norris in this volume.
The plan of the present article is as follows. In Section 2 we review the results in our own neighborhood, which is of special interest because of the relatively large amount of detailed information available for a sufficient number of stars to permit statistical discussions of such important topics as the age-metallicity and number-metallicity relations and primordial variations among different elements. In Section 3 we consider more large-scale trends, i.e. abundance gradients in the disk and halo of our Galaxy, the diffuse ISM, and molecular clouds. The remainder of the article is devoted to other galaxies, which raise special problems in abundance determination. The various indicators (statistical properties of stars, emission nebulae, and integrated light from stellar populations) and their virtues and limitations are discussed in Section 4, while in Section 5 we attempt to summarize what is known about Irregular (and Blue Compact) galaxies, Ellipticals, Dwarf Spheroidals, and Spirals, with special reference to abundance gradients, the nitrogen: oxygen ratio, and overall mass-metallicity relations. We give some brief conclusions and suggestions for future work in Section 6.