A temperature calibration for MK-class III giants from high-resolution spectral line-depth ratios

Abstract

A high‐resolution spectroscopic survey in the 6380–6460 Å region of 224 slowly‐rotating M‐K class III giants is presented. Spectral line‐depth ratios are calibrated against effective temperature obtained from B – V and V – I color indices in the range 3200–7500 K (M6–A9). A table of polynomial coefficients for 12 line‐ratio–Teff relations can be used to derive Teff of F–M stars to within 33 K (rms), and of early‐F and mid‐to‐late M stars to within 77–106 K (rms).

The metallicity effect on line-depth ratios in APOGEE H-band spectra

“…The line depths measured from the three profiles are similar to each other; the systematic differences between the depth measurement with the different profiles are approximately 1% for all the line pairs, and the standard deviations of the differences are typically of the order of 2%. Considering the conclusion by Strassmeier & Schordan (2000) and that the Gaussian-Hermite profile requires more points which may be more affected by blends, we use the depths measured by the parabola fitting in the following analysis unless otherwise mentioned. The impacts of blending lines around the selected lines will be discussed in Section 4.3.…”

“…However, most previous studies on the LDR method considered only optical spectra. For example, such works applied to giants, which is our targets, include: Strassmeier & Schordan (2000), Gray & Brown (2001), and Kovtyukh et al (2006). Recently, Fukue et al (2015) found nine LDR-T eff relations using H-band E-mail: mingjie@astron.s.u-tokyo.ac.jp spectra of eight stars (mainly giants), and Taniguchi et al (2018) found 81 relations using Y J-band spectra of nine giants.…”

Method to estimate the effective temperatures of late-type giants using line-depth ratios in the wavelength range 0.97–1.32 μm

“…For the 125 and 99 lines we selected, we used a quadratic function to fit three (or four) pixels near the bottom of each line rather than fitting the entire line profile, e.g., by the Gaussian or the Voigt function (Strassmeier & Schordan 2000). We define d…”

“…For the 125 and 99 lines we selected, we used a quadratic function to fit three (or four) pixels near the bottom of each line rather than fitting the entire line profile, e.g., by the Gaussian or the Voigt function (Strassmeier & Schordan 2000). We define d (n) i as a line depth from the continuum level to the bottom of the fitted function, where i indicates an ID number of the line and (n) indicates an ID number of a star.…”

The effect of surface gravity on line-depth ratios in the wavelength range 0.97–1.32 µm

“…It is still possible that the calibration data include systematic errors, but the relations can be easily improved once better (more precise and robust) effective temperatures of the calibrating stars become available. Following the pioneering studies of Gray (1989) and Gray & Johanson (1991), more than a hundred LDR relations were obtained for stars of different luminosity classes separately: dwarfs (Kovtyukh et al 2003(Kovtyukh et al , 2004, giants (Strassmeier & Schordan 2000;Gray & Brown 2001;Kovtyukh et al 2006a,b), and supergiants (Kovtyukh et al 1998;Kovtyukh & Gorlova 2000;Kovtyukh 2007). The uncertainties of the final T eff in the previous studies range from 5 to 30 K at best cases.…”