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Estimating precise metallicity of M dwarfs is a well-known difficult problemdue to their complex spectra. In this work, we empirically calibrate themetallicity using wide binaries with a F, G, or K dwarf and a M dwarfcompanion. With 1308 FGK+M wide binaries well observed by LAMOST, we calibratedM dwarf's [Fe/H] by using the Stellar LAbel Machine (SLAM) model, a data-drivenmethod based on support vector regression (SVR). The [Fe/H] labels of thetraining data are from FGK companions in range of [-1,0.5] dex. The Teffs areselected from Li et al. (2021), spanning [3100,4400] K. The uncertainties inSLAM estimates of [Fe/H] and Teff are ~0.15 dex and ~40 K, respectively, atsnri > 100, where snri is the signal-to-noise ratio (SNR) at i-band of M dwarfspectra. We applied the trained SLAM model to determine the [Fe/H] and Teff for~630,000 M dwarfs with low-resolution spectra in LAMOST DR9. Compared to otherliterature also using FGK+M wide binaries for calibration, our [Fe/H] estimatesshow no bias but a scatter of ~ 0.14-0.18 dex. However, the [Fe/H] compared toAPOGEE shows a systematic difference of ~ 0.10-0.15 dex with a scatter of ~0.15-0.20 dex. While the Teff compared to APOGEE has a bias of 3 K with ascatter of 62 K, it is systematically higher by 180 K compared to othercalibrations based on the bolometric temperature. Finally, we calculated thezeta index for 1308 M dwarf secondaries and presents a moderate correlationbetween zeta and [Fe/H].

Calibration of metallicity of LAMOST M dwarf stars Using FGK+M wide binaries