THE UBIQUITY OF THE RAPID NEUTRON-CAPTURE PROCESS*

<jats:title>ABSTRACT</jats:title> <jats:p>To better characterize the abundance patterns produced by the <jats:italic>r</jats:italic>-process, we have derived new abundances or upper limits for the heavy elements zinc (Zn, <jats:italic>Z</jats:italic>= 30), yttrium (Y, <jats:italic>Z</jats:italic>= 39), lanthanum (La, <jats:italic>Z</jats:italic>= 57), europium (Eu, <jats:italic>Z</jats:italic>= 63), and lead (Pb, <jats:italic>Z</jats:italic>= 82). Our sample of 161 metal-poor stars includes new measurements from 88 high-resolution and high signal-to-noise spectra obtained with the Tull Spectrograph on the 2.7 m Smith Telescope at the McDonald Observatory, and other abundances are adopted from the literature. We use models of the <jats:italic>s</jats:italic>-process in asymptotic giant branch stars to characterize the high Pb/Eu ratios produced in the <jats:italic>s</jats:italic>-process at low metallicity, and our new observations then allow us to identify a sample of stars with no detectable <jats:italic>s</jats:italic>-process material. In these stars, we find no significant increase in the Pb/Eu ratios with increasing metallicity. This suggests that <jats:italic>s</jats:italic>-process material was not widely dispersed until the overall Galactic metallicity grew considerably, perhaps even as high as [Fe/H] =−1.4, in contrast with earlier studies that suggested a much lower mean metallicity. We identify a dispersion of at least 0.5 dex in [La/Eu] in metal-poor stars with [Eu/Fe] <+0.6 attributable to the <jats:italic>r</jats:italic>-process, suggesting that there is no unique “pure” <jats:italic>r</jats:italic>-process elemental ratio among pairs of rare earth elements. We confirm earlier detections of an anti-correlation between Y/Eu and Eu/Fe bookended by stars strongly enriched in the <jats:italic>r</jats:italic>-process (e.g., CS 22892–052) and those with deficiencies of the heavy elements (e.g., HD 122563). We can reproduce the range of Y/Eu ratios using simulations of high-entropy neutrino winds of core-collapse supernovae that include charged-particle and neutron-capture components of <jats:italic>r</jats:italic>-process nucleosynthesis. The heavy element abundance patterns in most metal-poor stars do not resemble that of CS 22892–052, but the presence of heavy elements such as Ba in nearly all metal-poor stars without <jats:italic>s</jats:italic>-process enrichment suggests that the <jats:italic>r</jats:italic>-process is a common phenomenon.</jats:p>