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The Origin and Abundances of the Chemical Elements

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The Origin and Abundances of the Chemical Elements UC Irvine UC Irvine Previously Published Works Title The origin and abundances of the chemical elements Permalink https://escholarship.org/uc/item/86s95592 Journal Reviews of Modern Physics, 47(4) ISSN 0034-6861 Author Trimble, V Publication Date 1975 DOI 10.1103/RevModPhys.47.877 License https://creativecommons.org/licenses/by/4.0/ 4.0 Peer reviewed eScholarship.org Powered by the California Digital Library University of California The origin and abundances of the cheriiical eleriients*& Virginia Trimble~ Astronomy Program, University of Maryland, College Park, Maryland 20742 and Department of Physics, University of California, IrvineC, alifornia 92664 A NATO Advanced Studies Institute on the Origins and Abundances of the Chemical Elements was held at the Institute of Astronomy, Cambridge~ England from 23 July to 9 August 1974. The problems discussed there are the basis for this review. Three major areas can be identified. First, the normal distribution of chemical and isotopic abundances and deviations from it must be determined observationally. Second, the relevant nuclear processes and their sites and products must be identified. Third, a model of the evolution of galaxies which ixsgludes the relevant processes and produces the observed abundances must be formulated. There has been rapid progress recently in all three of these areas. CONTENTS F. Other Galaxies and QSOs 914 1. Helium 914 I. Int roduction and History 877 2. Mean metal abundance 915 II. Ob served Abundances 879 3. Abundance gradients in galaxies 915 A. The problem of the retrieval of meaningful data 879 III. Nuclear Processes, Their Sites and Products 917 1. D/H in molecules 879 A. Cosmological nucleosynthesis 917 2. The sola'r corona/photosphere iron discrepancy 879 B, Cosmic ray spallation and other versions of the x process 920 3. Weak He in QSOs and the galactic center 879 1. Supermassive stars 920 4. Peculiar A stars 880 2. Shell burning and shell flashes 920 5. Low energy solar flare particles 881 3. Novae 920 6. Galactic cosmic rays (as observed) 882 4. Supernova shock waves 920 Solar system abundances 882 5. Spallation by cosmic rays 921 1. The Earth —Moon system 882 C. Hydrostatic processes in stars 922 2. Meteorites 883 1. Hydrogen burning and the hot CNO cycle 922 a. The standard elemental abundance distribution 883 2. Helium burning 925 b. Abundances of the nuclides 887 3. Hydrostatic carbon, oxygen, and silicon burning 927 c. Helium, neon, carbon, and oxygen 887 D. Violent processes in stars 929 d. Xenonology and nucleocosmochronology 889 1. Explosive carbon burning 929 3. Solar photosphere and solar corona 891 a. Carbon detonation in degenerate cores 929 4. Solar wind and solar flares 895 b. Explosive carbon burning in massive stars 930 Other stars 896 2. Explosive oxygen and silicon burning and the e 1. Unevolved stellar compositions 896 process 932 a. Atmospheric compositions of individual stars 896 E. Processes which build heavy elements and rare isotopes 935 b. Stellar helium abundances 898 1. The s process 936 c. Stellar metal abundance as a function of age 899 2. The r process 942 d. The number of stars as a function of metal 3. The p process 948 abundance 901 F. Summing over the processes to get the yield per star and e. Supermetallicity 902 the yield per generation of heavy elements 950 2. Evolved stars 903 IV. The Evolution of Galaxies 953 a. Carbon stars and other giants 903 A. Observations of galaxies 953 b. Helium stars, R CrB variables, and other 1. Classification and morphology 954 hydrogen-de6cient stars 905 2. Luminosity distribution 954 c. Algol-type and other binaries 906 3. Colors and populations 955 d. White dwarfs 906 4. Masses 956 e. Sup ernovae 906 5. Gas mass and infall 956 D. The interstellar medium 907 6. Star formation rates 958 1. Hn regions 907 7. Supernova rates 960 2. Planetary nebulae 908 B. Homogeneous, one-zone models with constant IMF 961 3. Supernova remnants 90S C. Models with variable IMF 963 4. The general interstellar medium 909 D. Models with infall or inhomogeneity 964 E. Cosmic ray source composition{s) 910 E. Dynamical models of galactic evolution 964 1. The arriving and "source" compositions at 1. Spherical galaxies 964 intermediate energy 910 2. Disk galaxies 965 2 ~ Isotopic composition 913 3. Models with massive halos 966 3., The higher energy cosmic ray composition 914 V. Conclusions and Exhortations 967 4. The low energy cosmic ray composition 914 Acknowledgments 968 References 968 ~ The literature search for this review was terminated on 1 April 1975. f Dedicated to my father, Lyne Starling Trimble, vvho taught me I. INTRODUCTION AND HISTORY that, when you have a long story to tell, you must start at the beginning, until to the and then and to late keep going you get end, stop, my problems of determining the abundances of the mother, Virginia Farmer Trimble, who listened with great love and The patience to my early e8orts at story-telling. chemical elements and their isotopes and accounting for f Alfred P. Sloan I oundation Research Fellow 1972—1974. them in terms of a theory of nucleosynthesis and the Reviews of Modern Physics, Vol. 47, No. 4, October 1975 Copyright 1975 American Physical Society 878 Virginia Trimble: The origin and abundances of the chemical elements evolution of our Galaxy involves virtually every branch which show the least evidence of remelting after formationj of astronomy, and many related sciences, from objective are very similar to the solar ratios. This is even true to prism spectroscopy and neutron activation techniques to a lesser extent for terrestrial rocks, given that the sid- shell models of the nucleus and computerized histories of erophilic and chalcophilic elements are preferentially hidden the Galaxy. It is not perhaps surprising then that no in the core, and that volatile materials have-again been completely unique, clear picture of nucleosynthesis and lost. Moon rocks are similarly made up of only the more evolution has yet emerged. refractory elements, but again in standard proportions. Meteoritic, terrestrial, and lunar samples are particularly In order to be able to account for, the abundances of valuable because isotopic ratios (which are generally not the chemical elements, we must first know what they are. much affected by chemical fractionation) can be accurately The first attempt at compiling a table of "cosmic abun- determined in them, which is not generally possible from dances" (we will discuss shortly what this is supposed solar or stellar spectroscopy. Thus, it is possible to arrive to mean) was made by Goldschmidt (1937), who relied at a representative set of Solar System abundances, which primarily on data taken from meteorites and stellar spectra. (with a few exceptions) represents the surface of the Sun He distinguished elements as siderophilic, chalcophilic, or now, and which should represent the original solar nebula lithophilic, depending on whether they were found primarily out of which the Sun and planets were formed, in the in the metal, sulphide, or silicate phases of meteorites, and conventional view of things. adopted a very high abundance of Li, Be, and B on the basis of igneous rocks. In addition, abundances very similar to those in the Solar System are found in the interstellar gas in Hu regions Other important compilations were those of Brown around newly formed stars (called Hn regions because the (1949) and Suess and Urey (1956), who were able to make hydrogen there has been ionized by the ultraviolet radia- considerable improvements in combining the data from tion from the hot young stars) and in planetary nebulae various sources because of better understanding of the (the ejected outer. layers of older low mass stars). Among ratio of metal to silicate phases in meteorites and the the stars as well, abundances are quite similar, provided Earth, the ratio of volatile to refractory elements in the that we confine our attention to stars which (a) are not Sun and stars (which allows you to link up stellar and evolved off the main sequence so that their surfaces have meteorite data), and the ratios of isotopes which were by not been polluted by the products of their own nuclear then also regarded as containing important information. reactions, (b) have convective atmospheres or are rapidly rotating, so that their surface layers are kept well mixed, The current standard abundance distributions to which and (c) belong to Population I or the disk population of theoretical predictions are usually compared are those by stars which, on kinematic grounds, must have formed after Cameron (1968, 1973). The latter of these does not differ the first billion years or so of the life of our galaxy. Even very much from the former and is not regarded by every- the very old Population II stars, although their total one as being an improvement. In fact the standard abun- metal' abundance is very low, have those heavy elements dances have been remarkably stable. Only 8 elements out that can be detected present in nearly solar relative pro- of the 81 tabulated had abundances which changed by portions. The integrated spectra of other galaxies and factors of 3 or more between 1956 and 1968: C, Ga, and Pb quasars do not exhibit gross differences from "normal" were revised upward by factors of 3, 4, and 8.5, respec- abundances either. tively, and Be, La, Sm, Ta, and W downward by factors of 25, 4, 3, 3, and 3, respectively. Thus, it is possible to define a standard abundance distribution which applies to the Solar System and many The most surprising thing, in a way, is that such a other objects, and, then, to discuss how other objects deviate compilation is possible at all.
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