Abundance Analysis of Benchmark M Dwarfs

M dwarfs represent a substantial portion of the stellar population in the Milky Way. They are characterized by relatively low temperatures spanning 2500-3900 K and possess atmospheres abundant in molecules such as H2O, TiO, and CO, which are prominently featured in their optical spectra. High-resolution near-infrared (NIR) spectroscopy is a powerful tool for characterizing these stars, as molecular blending is significantly reduced in this spectral range. We analyzed high-resolution NIR spectra (R = 22,500) from the H-band APOGEE survey for 18 binary systems composed of G and M dwarfs. Using 1D LTE plane-parallel model atmosphere, we derived three effective temperature scales (T-A(Fe), T-A(O), and their average) as well as surface gravities (from spectral lines of Fe I, FeH, OH, and H2O) and individual chemical abundances for the elements C, O, Na, Mg, Al, Si, K, Ca, Ti, Cr, Mn, Fe, and Ni. We found excellent agreement in the metallicities between the G and M dwarfs, regardless of the adopted Teff scale, with a maximum abundance difference of δ([Fe/H]) = 0.04 ± 0.07 dex. No significant systematic offsets were identified between iron abundances derived from selected Fe I lines and those from FeH lines, suggesting that the adopted log gf values for FeH are accurate and that FeH lines are reliable Teff indicators. For temperature-sensitive elements (Mg, Si, Ca, Ti), the smallest abundance deviation, δ([Mg/H])(M - G) = -0.03 ± 0.08 dex, was obtained using the averaged Teff scale, indicating that this effective temperature scale is the most robust for chemical abundance analysis in M dwarfs from the H-band.