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

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∼ 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 µm and 2.32 µm 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 ef f 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 µm lines yields a reliable empirical relation at [F e/H] ≤ -0.4 dex, while a linear fit of any line offers a good metallicity scale for stars having [F e/H] ≥ 0.0 dex.