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

Abstract

Ratios of carefully selected line depths are sensitive to the stellar effective temperature (Teff). Relations established between the line-depth ratio (LDR) and Teff allow Teff to be determined precisely. LDRs can also, however, depend on metallicity and abundance ratios, which can limit the accuracy of the LDR method unless such effects are properly taken into account. We investigate the metallicity effect using H-band spectra and stellar parameters published by the APO Galactic Evolution Experiment (APOGEE) project. We clearly detect the effects of metallicity and abundance ratios: Teff derived from a given LDR depends on the metallicity, 100–800 K dex−1, and a dependence on the abundance ratios, 150–1000 K dex−1, also exists when the LDR involves absorption lines of different elements. For the 11 line pairs in the H- band that we investigated, the LDR–Teff relations with metallicity- and abundance-dependent terms added have scatters as small as 30–90 K within the range of 3700 < Teff < 5000 K and −0.7 < [Fe/H] < +0.4 dex. By comparing the observed spectra with synthetic ones, we find that saturation of the absorption lines at least partly explains the metallicity effect.