"The Evolution of Heavy Elements Since Big Bang"

G. J. Wasserburg
Caltech

Abstract

Pioneering work of Burbidge, Burbidge, Fowler and Hoyle and of A.G.W. Cameron showed that production of heavy nuclei required two mechanisms: neutron capture on a slow scale (s) and a rapid scale (r). Sites for the s-processes are well established from both stellar observations and stellar theory. The r-process sites are usually associated with SNe II and the formation of a neutron star. Abundances of radioactive r-nuclides in meteorites show that there had to be two distinct r-process sites. One site produced heavy r nuclei (Ba and above) at higher frequency as compared to the lower mass r nuclei. Early generation stars should thus show excesses of heavy r-nuclides. This has been observed in low metallicity halo stars (see Sneden et al. (2000) et seq.). Stars with low metallicity and very large enhancement of heavy elements must represent binaries and not the ISM. Intensive studies of several halo stars at [Fe/H] ~-3 show that all nuclei from Zn and below are not enriched, while heavy r nuclei may be greatly enriched. Thus SNe II   with extended envelopes cannot be the source of heavy r nuclei. J. Cohen et al. (ApJ 2003) discovered a halo star with enormous enrichment of both s and r process nuclides. This led to a new proposal that s process and heavy r process enrichment is the result of the evolution of a binary star system with accretion induced collapse (AIC) of the white dwarf to form a neutron star (QW ApJ 2003). If the mechanism of AIC and binaries is the source of the r-process, it leads to a high production rate of neutron stars in the galaxy (10-20 per century) with implications for neutrino detection. The Qian-Wasserburg model of heavy element enrichment will be outlined, including the initial production of elements by VMS until [Fe/H] @ -3 to produce the “prompt inventory” of metals. This is sufficient to cause the reionization (Bromm et al. (2001); Oh et al. 2001). This state was followed by production of normal stars, AIC and SNe II. SNIa became active after [Fe/H] ~ -1. Quantitative predictions of elemental abundances in low metallicity  stars will be compared to observational data. The inventory of lower mass heavy nuclei in Damped Ly a clouds, and the Ly a forest will be related to this model and the composition of the IGM discussed relative to the prompt inventory (Qian, Sargent, Wasserburg, ApJLett, 2002).