Chemical Compositions of Evolved Stars From Near-Infrared IGRINS High-Resolution Spectra. I. Abundances in Three Red Horizontal Branch Stars

TL;DR

This study derives detailed elemental abundances for three red horizontal branch stars using high-resolution near-infrared spectra, including first-time measurements of sulfur, phosphorus, and potassium, enhancing understanding of stellar and galactic chemical evolution.

Contribution

It presents the first determination of S, P, and K abundances in these stars using IGRINS near-infrared spectra, demonstrating the method's advantages over optical spectroscopy for certain elements.

Findings

First measurements of S, P, and K in these stars.

Near-infrared spectra provide more consistent elemental abundances.

Comparison shows IR spectra yield more reliable results for some elements.

Abstract

We have derived elemental abundances of three field red horizontal branch stars using high-resolution (R$\simeq$ 45,000), high signal-to-noise ratio (S/N $\gtrsim$ 200) $H$ and $K$ band spectra obtained with the Immersion Grating Infrared Spectrograph (IGRINS). We have determined the abundances of 21 elements including $\alpha$ (Mg, Si, Ca, S), odd-Z (Na, Al, P, K), Fe-group (Sc, Ti, Cr, Co, Ni), neutron-capture (Ce, Nd, Yb), and 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 (2$-$0) and (3$-$1) $^{12}$CO and (2$-$0) $^{13}$CO lines near 23440 \AA\ and $^{13}$CO (3$-$1) lines at about 23730 \AA. 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 atomic lines of the same species for several elements, especially the $\alpha$ elements. This in turn provides more reliable abundances for the elements with analytical difficulties in the optical spectral range.