$H$-band Temperature and Metallicity Indicators for Cool Giants: Empirical Relations in Bayesian Framework

Abstract

We explored here the near-infrared $H$-band atmospheric window aiming to provide quantitative diagnostic tools for deriving stellar parameters, for instance, effective temperature ($T_{eff}$) and metallicity ([$Fe/H$]), of cool giants ($T_{eff}$ $<$ 5000 K) using low-resolution spectra. We obtained 177 cool giants from the X-shooter spectral library covering a wider metallicity range ($-$2.35 dex $<$ [$Fe/H$] $<$ 0.5 dex) than in earlier works. Degrading the spectral resolution to R$\sim$ 1200, we estimated equivalent widths of several important spectral features, and the behavior of spectral features with stellar parameters are studied. Also, the empirical relations for deriving $T_{eff}$ and [$Fe/H$] are established in the Bayesian framework. We found that $^{12}$CO at 1.56 $\mu$m and 1.62 $\mu$m, and $^{12}$CO+MgI at 1.71 $\mu$m are the best three $T_{eff}$ indicators with a typical accuracy of 153 K, 123 K and 107 K, respectively. The cubic Bayesian model provides the best metallicity estimator with a typical accuracy of 0.22 dex, 0.28 dex, and 0.24 dex for FeH at 1.62 $\mu$m, $^{12}$CO at 1.64 $\mu$m, and Fe I at 1.66 $\mu$m, respectively. We also showed a detailed quantitative metallicity dependence of $T_{eff}-$EWs correlations defining three metallicity groups, supersolar ([$Fe/H$] $>$ 0.0 dex), solar ($-$0.3 dex $<$ [$Fe/H$] $<$ 0.3 dex), and subsolar ([$Fe/H$] $<$ $-$0.3 dex), from Hierarchical Bayesian modelling. The difference between the solar and subsolar relationship is statistically significant, but such difference is not evident between the solar and supersolar groups.