Calibration of metallicity of LAMOST M dwarf stars using FGK plus M wide binaries

Estimating precise metallicity of M dwarfs is a well-known difficult problem due to their complex spectra. In this work, we empirically calibrate the metallicity using wide binaries with a F, G, or K dwarf and a M dwarf companion. With 1308 FGK+M wide binaries well observed by LAMOST, we calibrated M dwarf's [Fe/H] by using the Stellar LAbel Machine (SLAM) model, a data-driven method based on support vector regression. The [Fe/H] labels of the training data are from FGK companions in range of [-1,0.5] dex. The T(eff)s are based on APOGEE, spanning [3100, 4400] K. The uncertainties in SLAM estimates of [Fe/H] and T-eff are similar to 0.15 dex and similar to 40 K, respectively, at snri >100, where snri is the signal-to-noise ratio (SNR) at i band of M dwarf spectra. We applied the trained SLAM model to determine the [Fe/H] and T-eff for similar to 630 000 M dwarfs with low-resolution spectra in LAMOST DR9. Compared to other literature also using FGK+M wide binaries for calibration, our [Fe/H] estimates show no bias but a scatter of similar to 0.14-0.18 dex. However, the [Fe/H] compared to APOGEE shows a systematic difference of similar to 0.10-0.15 dex with a scatter of similar to 0.15-0.20 dex. While the T-eff compared to APOGEE has a bias of 3 K with a scatter of 62 K, it is systematically higher by 180 K compared to other calibrations based on the bolometric temperature. Finally, we calculated the zeta index for 1308 M dwarf secondaries and presents a moderate correlation between zeta and [Fe/H].