Full metal bracket: A calibration of infrared and optical spectroscopic metallicities of M dwarfs over 1.5 dex



he determination of metallicities in late-type stars is particularly difficult because their cool atmospheres enable the formation of complex molecules. Instead, most studies use wide binaries that include a solar-type primary and an M dwarf companion. However, most previous studies utilize only a small number (15–25) of binaries, limiting the effective range of their calibrations. We present our observations and analysis of a sample of 105 FGK-M wide binary systems with metallicities of –1.04 < [Fe/H] < +0.56 and companion spectral types from K5.5 to M6. We obtain moderate resolution (1300 < R < 2000) visible and near-infrared spectra for each late-type companion, and high-resolution spectra for the 44 primary stars that lack published metallicities. We use our sample to determine, empirically, which spectral features are best correlated with the metallicity of the primary. We find over 80 features in K and M dwarf visible and near-infrared spectra that are useful for predicting metallicity. We use a subset of these features to derive metallicity calibrations for different wavelength regimes, and show that it is possible to get metallicities reliable to < 0.10 dex using either visible, J, H, or K band spectra, however, when using visible spectra this requires the use of different calibrations for early-type (K5.5 to M2) and late-type (M2 to M6) dwarfs. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)