Chemical Compositions of Evolved Stars from Near-infrared IGRINS High-resolution Spectra. I. Abundances in Three Red Horizontal Branch Stars

Abstract

We have derived elemental abundances of three field red horizontal branch stars using high-resolution ( R  ≃ 45,000), high signal-to-noise ratio (S/N ≳ 200) H - and K -band spectra obtained with the Immersion Grating Infrared Spectrograph (IGRINS). We have determined the abundances of 21 elements, including α (Mg, Si, Ca, S), odd- Z (Na, Al, P, K), Fe-group (Sc, Ti, Cr, Co, Ni), neutron-capture (Ce, Nd, Yb), CNO-group elements. S, P, and K are determined for the first time in these stars. H - and K -band spectra provide a substantial number of S i lines, which potentially can lead to a more robust exploration of the role of sulfur in the cosmochemical evolution of the Galaxy. We have also derived 12 C/ 13 C ratios from synthetic spectra of the first-overtone 12 CO (2–0) and (3–1) and 13 CO (2–0) lines near 23440 Å and 13 CO (3–1) lines at about 23730 Å. Comparison of our results with the ones obtained from the optical region suggests that the IGRINS high-resolution H - and K -band spectra offer more internally self-consistent line abundances of the same species for several elements, especially the α -elements. This in turn provides more reliable abundances for the elements with analytical difficulties in the optical spectral range.