To better characterize the abundance patterns produced by the r -process , we have derived new abundances or upper limits for the heavy elements zinc ( Zn , Z = 30 ) , yttrium ( Y , Z = 39 ) , lanthanum ( La , Z = 57 ) , europium ( Eu , Z = 63 ) , and lead ( Pb , Z = 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 McDonald Observatory , and other abundances are adopted from the literature .
We use models of the s -process in AGB stars to characterize the high Pb/Eu ratios produced in the s -process at low metallicity , and our new observations then allow us to identify a sample of stars with no detectable s -process material .
In these stars , we find no significant increase in the Pb/Eu ratios with increasing metallicity .
This suggests that s -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 r -process , suggesting that there is no unique “ pure ” r -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 r -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 r -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 s -process enrichment suggests that the r -process is a common phenomenon .