Neutron-Capture Elements in the Early Galaxy: Insights from a Large Sample of Metal-poor Giants

New abundances for neutron-capture (n-capture) elements in a large sample of metal-poor giants from the Bond survey are presented. The spectra were acquired with the KPNO 4 m echelle and coudé feed spectrographs, and have been analyzed using LTE fine-analysis techniques with both line analysis and spectral synthesis. Abundances of eight n-capture elements (Sr, Y, Zr, Ba, La, Nd, Eu, and Dy) in 43 stars have been derived from blue (λλ4070-4710, R ~ 20,000, S/N ratio ~ 100-200) echelle spectra and red (λλ6100-6180, R ~ 22,000, S/N ratio ~ 100-200) coudé spectra, and the abundance of Ba only has been derived from the red spectra for an additional 27 stars.

Overall, the abundances show clear evidence for a large star-to-star dispersion in the heavy element-to-iron ratios. This condition must have arisen from individual nucleosynthetic events in rapidly evolving halo progenitors that injected newly manufactured n-capture elements into an inhomogeneous early Galactic halo interstellar medium. The new data also confirm that at metallicities [Fe/H] ≲ -2.4, the abundance pattern of the heavy (Z ≥ 56) n-capture elements in most giants is well-matched to a scaled solar system r-process nucleosynthesis pattern.

The onset of the main r-process can be seen at [Fe/H] ≈ -2.9; this onset is consistent with the suggestion that low mass Type II supernovae are responsible for the r-process. Contributions from the s-process can first be seen in some stars with metallicities as low as [Fe/H] ~ -2.75 and are present in most stars with metallicities [Fe/H] > -2.3. The appearance of s-process contributions as metallicity increases presumably reflects the longer stellar evolutionary timescale of the (low-mass) s-process nucleosynthesis sites.

The lighter n-capture elements (Sr-Y-Zr) are enhanced relative to the heavier r-process element abundances. Their production cannot be attributed solely to any combination of the solar system r- and main s-processes, but requires a mixture of material from the r-process and from an additional n-capture process that can operate at early Galactic time. This additional process could be the weak s-process in massive (~25 M_☉) stars, or perhaps a second r-process site, i.e., different from the site that produces the heavier (Z ≥ 56) n-capture elements.