Meyer and Zinner: Nucleosynthesis 69 Nucleosynthesis Bradley S. Meyer Clemson University Ernst Zinner Washington University Primitive meteorites contain nano- to micrometer-sized dust grains with large isotopic anoma- lies relative to the average solar system composition. These large anomalies convincingly demon- strate that the dust grains condensed in the outflow from their parent stars and survived destruc- tion in the interstellar medium and solar-system processing to retain memory of the astrophysical setting of their formation. In addition to the dust grains, certain primitive refractory inclusions in meteorites also show isotopic anomalies, which suggests that the inclusions formed in the solar system from large collections of anomalous dust. Finally, the inferred presence of short-lived radioisotopes in the early solar nebula yields important clues about the circumstances of the Sun’s birth. We review these topics in cosmochemistry and their implications for our ideas about stellar nucleosynthesis. 1. INTRODUCTION AND HISTORY different chondrite groups are considered to reflect varia- tions in the environment where their parent asteroids formed Today it is well established that all elements from C on (Palme, 2000). However, major elemental differentiation up were made by stellar nucleosynthesis. After the Big Bang occurred through planet formation and geological processes the universe consisted only of H and He and trace amounts taking place on larger planetary bodies. Thus, terrestrial of Li, Be, and B, and it was not until the formation of the rocks have elemental compositions that are quite different first stars that the heavier elements came into existence. from the overall mix from which the whole SS was made. There exists some evidence now (e.g., Umeda and Nomoto, In contrast, primitive meteorites to a large extent preserve 2003) that the earliest stars, which started out with essential- the original composition of the SS. They originate from ly only H and He, were very heavy. Such stars run through small asteroids that did not experience any planetary dif- their evolution relatively rapidly (in millions of years) and ferentiation. Their elemental abundances agree very well produce nearly all the heavier elements by various nuclear with that of the Sun except for the volatile elements H, C, processes in their hot interiors. They end their lives in gigan- N, and O, the noble gases, and Li (Anders and Grevesse, tic explosions as supernovae that expel the newly synthe- 1989; Grevesse et al., 1996). Since geological processes do sized elements in their ejecta into the interstellar medium not affect isotopic ratios very much (they introduce only (ISM), where they are mixed with interstellar gas. The next some mass-dependent fractionation), the isotopic compo- generation of stars incorporated these elements during their sitions of solar materials from different planetary bodies formation and the material in our galaxy underwent several (Earth, Moon, Mars, and the parent bodies of different types such cycles before our solar system (SS) formed. In con- of meteorites) are essentially identical. An exception are trast to massive stars that become supernovae (SNe), stars isotopic anomalies in certain elements that are due to the with a mass less than ~8 M experience a different history. decay of radioactive isotopes. Some of these radioisotopes They take hundreds of millions to billions of years to burn were short-lived, i.e., they existed only early in the SS and the H and He in their interior into C and O and in their late evidence for their decay products are found in early-formed stages lose their outer envelopes in the form of stellar winds SS solids. and planetary nebulae, thereby leaving behind a C-O white Although stellar nucleosynthesis had been proposed be- dwarf (WD) star that does not undergo further nuclear pro- fore (Hoyle, 1946), in the early 1950s it was far from clear cesses. The stellar wind and planetary nebula material are that most elements are synthesized in stars. The first obser- enriched in certain nuclei that were produced in hot shells vational evidence for stellar nucleosynthesis came from the on top of the WD. discovery of the unstable element Tc in the spectra of S- When the SS formed it incorporated material from many stars (Merrill, 1952). A few years later in their classical pa- different stellar sources: supernovae, late-type stars, and pers, Burbidge et al. (1957) and Cameron (1957) estab- novae. This material was mixed extremely well so that the lished the theoretical framework for stellar nucleosynthesis. SS as a whole has a very uniform elemental and isotopic They proposed a scheme of eight nucleosynthetic processes composition. Differences in elemental abundances among taking place in different stellar sources under different con- 69 70 Meteorites and the Early Solar System II Fig. 1. (a) Abundances of the elements in the solar system (Anders and Grevesse, 1989). (b) Abundances of the nuclides of a given element produced by the p-, r-, and s-process, respectively. ditions. One important motivation for this work was to ex- SS ratios, in the element H (Boato, 1954) and the noble plain regularities in the abundance of the nuclides in the SS gases Xe (Reynolds, 1960a) and Ne (Black and Pepin, 1969). (Fig. 1) as obtained by the study of meteorites (Suess and However, these early signs were largely ignored, and it was Urey, 1956). (For a table of the solar system abundances of not until the discovery of an 16O excess (Clayton et al., the nuclides and a brief description of their principal nucleo- 1973), an isotopic anomaly in a major rock-forming element, synthesis production processes, the reader is invited to visit in refractory inclusions (Ca-Al-rich inclusions, or CAIs) that the Web site http://nucleo.ces.clemson.edu/pages/solar_ the idea of the survival of presolar isotopic signatures was abundances/.) While the SS represents only a grand aver- widely accepted. This discovery was followed by the dis- age of many distinct stellar sources, the SS (sometimes also covery of anomalies in elements such as Ca, Ti, Cr, Fe, Zn, termed cosmic) abundances of the elements and isotopes Sr, Ba, Nd, and Sm (for an overview see Lee, 1979, 1988; (Cameron, 1973; Anders and Grevesse, 1989; Grevesse et Wasserburg, 1987; Clayton et al., 1988). Important was the al., 1996; Lodders, 2003) became an important reference realization that certain isotopic patterns represented distinct for astronomers. They remained the touchstone for testing nucleosynthetic components. For example, Ba, Nd, and Sm models of stellar nucleosynthesis as two generations of nu- in the FUN inclusion EK1-4-1 from the Allende meteorite clear astrophysicists tried to reproduce them (e.g., Käppeler (McCulloch and Wasserburg, 1978a,b) show an r-process et al., 1989; Timmes et al., 1995). The complete homogeni- signature and thus provided direct evidence for the distinct zation of all presolar material in a hot solar nebula (Cam- origin of the r- and s-process nuclides (see also Begemann, eron, 1962) seemed to be confirmed by the observation that 1993). Another example is the correlated excesses and de- the isotopic compositions of SS materials, including primi- pletions of the neutron-rich (n-rich) isotopes 48Ca and 50Ti in tive meteorites, are very uniform. The prevalent view was the FUN inclusions EK1-4-1 and C-1 and in hibonite grains that all solids had been vaporized and thus had lost all mem- (Lee et al., 1978; Niederer et al., 1980; Fahey et al., 1985; ory of their stellar sources, and that the solids we find now Zinner et al., 1986) that indicate a separate nucleosynthetic in primitive meteorites were produced by condensation from process for the production of the n-rich isotopes of the Fe- a well-mixed gas (Grossman, 1972). peak elements. While some of these isotopic anomalies can The first hints that not all presolar material had been be fairly large — 50Ti excesses in hibonite grains range up homogenized and some components found in primitive me- to almost 30% (Ireland, 1990) — it has to be emphasized teorites preserved a record of their presolar history came that the solids exhibiting these anomalies are of SS origin in the form of isotopic anomalies, i.e., deviations from the and only incorporated certain presolar components. This is Meyer and Zinner: Nucleosynthesis 71 not only evidenced by the size of the effects that are gen- dissolution and physical separation procedures in order to erally much smaller than those expected for bona fide stel- track the gases’ solid carriers (Amari et al., 1994). This ap- lar material (Begemann, 1993), but also by the fact that proach, termed “burning down the haystack to find the nee- other elements (e.g., O, Mg, Si) in CAIs carrying isotopic dle,” resulted in the discovery of presolar diamond, the car- anomalies in certain elements have essentially normal iso- rier of Xe-HL (Lewis et al., 1987); silicon carbide (SiC), the topic compositions. carrier of Ne-E(H) and Xe-S (Bernatowicz et al., 1987; Tang In addition to isotopic anomalies of nucleosynthetic ori- and Anders, 1988b); and graphite, the carrier of Ne-E(L) gin, solids from primitive meteorites show isotopic effects (Amari et al., 1990). Subsequently, other presolar grain due to the decay of short-lived, now extinct radioisotopes. types, not tagged by exotic noble gases, were identified by The first evidence for the presence of a short-lived isotope at isotopic analysis of individual grains in the ion microprobe. 129 the time of SS formation came from excess Xe (Reynolds, These types include silicon nitride (Si3N4); oxides such as 1960b) and its correlation with 127I (Jeffery and Reynolds, corundum, spinel, and hibonite; and silicates. With the ex- 129 1961), indicating the decay of I (T1/2 = 16 m.y.).