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

Abstract

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.