The R-Process Alliance: A Comprehensive Abundance Analysis of HD 222925, a Metal-poor Star with an Extreme R-process Enhancement of [Eu/H] = −0.14*

Ian U. Roederer1,2, Charli M. Sakari3, Vinicius M. Placco2,4, Timothy C. Beers2,4, Rana Ezzeddine2,5, Anna Frebel2,5, and Terese T. Hansen6
Astrophysical Journal 865, 129 Link to Article [DOI: 10.3847/1538-4357/aadd92]
1Department of Astronomy, University of Michigan, 1085 S. University Ave., Ann Arbor, MI 48109, USA
2Joint Institute for Nuclear Astrophysics—Center for the Evolution of the Elements (JINA-CEE), USA
3Department of Astronomy, University of Washington, Seattle, WA 98195-1580, USA
4Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA
5Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
6Carnegie Observatories, Pasadena, CA 91101, USA

We present a detailed abundance analysis of the bright (V = 9.02), metal-poor ([Fe/H] = −1.47 ± 0.08) field red horizontal-branch star HD 222925, which was observed as part of an ongoing survey by the R-Process Alliance. We calculate stellar parameters and derive abundances for 46 elements based on 901 lines examined in a high-resolution optical spectrum obtained using the Magellan Inamori Kyocera Echelle spectrograph. We detect 28 elements with 38 ≤ Z ≤ 90; their abundance pattern is a close match to the solar r-process component. The distinguishing characteristic of HD 222925 is an extreme enhancement of r-process elements ([Eu/Fe] = +1.33 ± 0.08, [Ba/Eu] = −0.78 ± 0.10) in a moderately metal-poor star, so the abundance of r-process elements is the highest ([Eu/H] = −0.14 ± 0.09) in any known r-process-enhanced star. The abundance ratios among lighter (Z ≤ 30) elements are typical for metal-poor stars, indicating that production of these elements was dominated by normal Type II supernovae, with no discernible contributions from Type Ia supernovae or asymptotic giant branch stars. The chemical and kinematic properties of HD 222925 suggest it formed in a low-mass dwarf galaxy, which was enriched by a high-yield r-process event before being disrupted by interaction with the Milky Way.

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