The R-Process Alliance: Chemical Abundances for a Trio of r-process-enhanced Stars—One Strong, One Moderate, and One Mild*

摘要

We present detailed chemical abundances of three new bright (V ~ 11), extremely metal-poor ([Fe/H] ~ ?3.0), r-process-enhanced halo red giants based on high-resolution, high-S/N Magellan/MIKE spectra. We measured abundances for 20–25 neutron-capture elements in each of our stars. J1432?4125 is among the most r-process-rich r-II stars, with [Eu/Fe]?=?+1.44?±?0.11. J2005?3057 is an r-I star with [Eu/Fe]?=?+0.94?±?0.07. J0858?0809 has [Eu/Fe]?=?+0.23?±?0.05 and exhibits a carbon abundance corrected for an evolutionary status of [C/Fe]corr?=?+0.76, thus adding to the small number of known carbon-enhanced r-process stars. All three stars show remarkable agreement with the scaled solar r-process pattern for elements above Ba, consistent with enrichment of the birth gas cloud by a neutron star merger. The abundances for Sr, Y, and Zr, however, deviate from the scaled solar pattern. This indicates that more than one distinct r-process site might be responsible for the observed neutron-capture element abundance pattern. Thorium was detected in J1432?4125 and J2005?3057. Age estimates for J1432?4125 and J2005?3057 were adopted from one of two sets of initial production ratios each by assuming the stars are old. This yielded individual ages of 12?±?6 Gyr and 10?±?6 Gyr, respectively.