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20. Meteorites and the Chemical Evolution of the Milky Nittler and Dauphas: Meteorites and the Chemical Evolution of the Milky Way 127 Meteorites and the Chemical Evolution of the Milky Way Larry R. Nittler Carnegie Institution of Washington Nicolas Dauphas The University of Chicago The theory of galactic chemical evolution (GCE) describes how the chemical and isotopic composition of galaxies changes with time as succeeding generations of stars live out their lives and enrich the interstellar medium with the products of nucleosynthesis. We review the basic astronomical observations that bear on GCE and the basic concepts of GCE theory. In addition to providing a standard set of abundances with which to compare GCE predictions, meteorites also provide information about how the galaxy has evolved through the study of preserved pre- solar grains and radioactive isotopes. 1. INTRODUCTION history, we concentrate on recent developments of the field. The reader is referred to the excellent books by Pagel (1997) The solar system is situated in the disk of the Milky Way and Matteucci (2003) for comprehensive reviews of the rich galaxy, some 8.5 kiloparsecs from the galactic center. It astronomical literature on the topic. formed 4.567 G.y. ago (Amelin et al., 2002) and its com- position represents a snapshot of the composition of the 2. BASIC CONCEPTS AND Milky Way in the solar neighborhood at that time. The bulk ASTRONOMICAL CONSTRAINTS composition of the solar system has often been referred to as the “cosmic composition” (Anders and Grevesse, 1989), Excluding dark matter, the Milky Way consists of sev- with the underlying assumption that it represents the aver- eral components, including a thin disk (to which the solar age composition of the galaxy. However, stellar nucleosyn- system belongs), a thick disk, a bright inner bulge, and a thesis is a phenomenon that is discrete in both time and large spherical diffuse stellar halo. Just how these compo- space. The concept of cosmic composition therefore breaks nents formed is not known, but there are several competing down when the chemical and isotopic composition of the models (Matteucci, 2003). Most likely, both primordial col- galaxy is examined at a fine scale and astronomy abounds lapse of a protogalactic gas cloud and subsequent accretion with examples of objects and environments with nonsolar and merger of smaller systems have played a role (Gibson compositions. Galactic chemical evolution (GCE) is the et al., 2003). However, many aspects of GCE modeling do name given to the theory of how the chemical composition not depend strongly on the overall model of galactic forma- of a galaxy varies with time and space as succeeding gener- tion. Because the solar system belongs to the thin disk, we ations of stars live out their lives and enrich the interstellar will primarily concentrate on observations and GCE models medium (ISM) with the products of nucleosynthesis. Note of this component. that the word “chemical” is somewhat misleading in this A crucial quantity involved in any discussion of GCE is context, since GCE refers only to the abundances of nuclei metallicity, which is the abundance of elements heavier than in the galaxy, not to the chemical state in which they might He (“metals” for astronomers). The letter Z is usually used to appear. indicate the total metallicity. However, the normalized Fe In this chapter, we discuss some of the ways in which abundance ([Fe/H] = log(Fe/H)–log(Fe/H) ) is often used as meteorites can help unravel the presolar nucleosynthetic his- a proxy for total metallicity, as Fe is relatively easy to meas- tory of the Milky Way. A key constraint for models of GCE ure in a large number of astronomical environments. The has long been the solar isotopic and elemental abundance metallicity of the Sun (Z ) has long been thought to be ~2%, pattern, largely determined by measurements of CI chon- but a recent downward revision in the solar O abundance drites. However, meteorites also preserve a record of GCE in by Allende Prieto et al. (2001) now indicates that it is closer the form of preserved presolar dust grains and extinct radio- to 1.4%. activities. Here, we will review the basic concepts of GCE Descriptions and models of GCE require a number of theory and astronomical constraints before considering the key ingredients: role of GCE in determining the isotopic compositions of 1. Boundary conditions: The initial composition (often presolar grains and the abundances of radioactivities in the taken to be that generated by the Big Bang) and whether the early solar system. Although the subject of GCE has a rich system is closed or open must be defined. 127 128 Meteorites and the Early Solar System II 2. Stellar yields: The abundances of isotopes produced by nucleosynthesis in stars of various types are required. These are determined by stellar evolutionary calculations coupled to nuclear reaction networks (e.g., Meyer and Zin- ner, 2006). In general, the nucleosynthesis abundance pat- terns ejected by stars depend critically on the stellar mass and metallicity. Moreover, there are still large uncertainties in the predicted yields due to uncertainties in the stellar evo- lution physics and nuclear reaction cross sections. A key concept related to stellar yields is the definition of primary and secondary species. A primary specie is one that can be synthesized in a zero-metallicity star, consisting initially of pure H and He. Examples of primary species are 16O and 12C, both made by stellar He burning. In contrast, nucleo- synthesis of a secondary specie requires some preexisting metals to be present in the star. Some secondary species in- clude the heavy isotopes of O, 14N, and the s-process ele- Fig. 1. Closed-box linear model with instantaneous recycling ments (Meyer and Zinner, 2006). approximation. The gas surface density is the mass of gas con- 3. The star formation rate (SFR) is usually parameter- tained in a cylinder divided by the area of the top surface of this ized in GCE models. A very common parameterization is cylinder. The rate of star formation is assumed to scale linearly to assume that the SFR is proportional to the disk gas sur- with the gas density (linear model) The galactic disk is assumed σ Ψ ∝ σn to have been isolated for most of its history (closed-box model). face density ( g) to some power: , where n = 1–2 (Schmidt, 1959). However, there are many examples of more complicated expressions for the SFR (e.g., Dopita and Ryder, 1994; Wyse and Silk, 1989). 2. Age-metallicity relationship (AMR): The measured 4. The initial mass function [φ(m)] describes the num- metallicity of stars decreases on average with stellar age ber distribution of stars that form in a given mass interval at (Twarog, 1980; Edvardsson et al., 1993), as would be ex- a given time. It is usually parameterized as a single- or multi- pected from the basic idea of GCE. However, there is a large component power law; for example, the common Salpeter observed scatter in metallicity, greater than a factor of 2, (1955) IMF is: φ(M) ∝ M–2.35. Other parameterizations for stars of a given age in the solar neighborhood (Edvards- have different power-law indices for different mass ranges son et al., 1993; Rocha-Pinto et al., 2000). This scatter is (Scalo, 1986). In most models it is assumed that the IMF is still not well-explained and makes the AMR a rather weak constant in space and time; this is consistent with observa- constraint for GCE models. tional evidence (Kroupa, 2002). 3. Abundance ratio evolution: Because different ele- 5. Except for the simplest models (see next section), ments are formed by different nucleosynthetic processes, infall of gas from the halo onto the disk and outflows from they can evolve at different rates. Thus, studies of element galactic winds are often included in GCE models. The infall abundance ratios as a function of metallicity can provide rate is usually assumed to vary with galactocentric radius important information about GCE (McWilliam, 1997). For and decrease with time. There is evidence for at least two example, low-metallicity stars have higher-than-solar ratios independent episodes of infall leading to the formation of the of so-called α elements (e.g., 16O, 24Mg) to Fe, but these ra- halo and the thin disk respectively (Chiappini et al., 1997). tios decrease to solar as solar [Fe/H] is reached. This reflects Galactic chemical evolution models are constrained by the fact that the α elements are made primarily in Type II a number of disparate observational data. These include supernovae, which evolve rapidly (<107 yr), whereas a ma- the present-day values of the star formation and supernova jor fraction of Fe in the galaxy is made by Type Ia super- rates, the present-day values of the surface mass density of novae, which evolve on much longer timescales (~109 yr). stars and gas (Fig. 1), stellar mass function, and gas infall Thus, high α/Fe ratios at low metallicities indicate that Type Ia rate, as well as chemical abundances measured in a wide supernovae had not yet had time to evolve and enrich the variety of stars and interstellar gas. The abundance data can ISM with their ejecta. The exact shape of abundance trends be divided into a number of constraints: are determined by the relative fractions and timescales of 1. Solar abundances: The composition of the Sun (e.g., Type Ia and II supernovae as well as other details of GCE. Anders and Grevesse, 1989), determined both by spectros- 4. G-dwarf metallicity distribution: G dwarfs are low- copy of the Sun and analysis of CI chondrites, represents a mass stars (~0.9–1.1 M ) that have lifetimes greater than or sample of the ISM 4.6 G.y.
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