New Temperature and Metallicity Scale of Cool Giants from K-band Spectra

TL;DR

This paper develops new empirical spectral diagnostics using low-resolution K-band spectra of 260 cool giants to accurately determine their effective temperature and metallicity across a wide metallicity range.

Contribution

It introduces a comprehensive set of empirical relations for stellar parameters based on K-band spectral features, extending previous work to cover a broader metallicity spectrum.

Findings

Empirical relations for T_eff using CO bands are established.

Metallicity-dependent spectral line relations are quantified.

Reliable metallicity scale is provided for stars with different [Fe/H] ranges.

Abstract

We present here quantitative diagnostic tools for cool giants that employ low-resolution near-infrared spectroscopy in the $K$-band for stellar population studies. In this study, a total of 260 cool giants (177 stars observed with X-shooter and 83 stars observed with NIFS) are used covering a wider metallicity range than in earlier works. We measure equivalent widths of some of the selected important $K$-band spectral features like Na I, Fe I, and $^{12}$CO after degrading the spectral resolution (R $\sim$ 1200) to investigate the spectral behavior with fundamental parameters (e.g. effective temperature and metallicity). We derive empirical relations to measure effective temperature using the $^{12}$CO first-overtone band at 2.29 $\mu$m and 2.32 $\mu$m and show a detailed quantitative metallicity dependence of these correlations. We find that the empirical relations based on solar-neighborhood stars can incorporate large uncertainty in evaluating $T_{eff}$ for metal-poor or metal-rich stars. Furthermore, we explore all the spectral lines to establish the empirical relation with metallicity and find that the quadratic fit of the combination of Na I and $^{12}$CO at 2.29 $\mu$m lines yields a reliable empirical relation at [$Fe/H$] $\leq$ $-$0.4 dex, while a linear fit of any line offers a good metallicity scale for stars having [$Fe/H$] $\geq$ 0.0 dex.