Stellar Characterization and Chemical Abundances of Exoplanet Hosting M Dwarfs from APOGEE Spectra: Future JWST Targets

Exoplanets hosting M dwarfs are the best targets to characterize Earth-likeor super-Earth planetary atmospheres with the James Webb Space Telescope(JWST). We determine detailed stellar parameters (T_ eff, logg, andξ) and individual abundances of twelve elements for four cool M dwarfshosting exoplanets TOI-1685, GJ 436, GJ 3470, and TOI-2445, scheduled forfuture observations by the JWST. The analysis utilizes high-resolutionnear-infrared spectra from the SDSS-IV APOGEE survey between 1.51-1.69μ.Based on 1D-LTE plane-parallel models, we find that TOI-2445 is slightlymetal-poor ([Fe/H] = -0.16±0.09), while TOI-1685, GJ 436 and GJ 3470 aremore metal-rich ([Fe/H] = 0.06±0.18, 0.10±0.20 dex, 0.25±0.15). Thederived C/O ratios for TOI-2445, TOI-1685, GJ 436, and GJ 3470 are0.526±0.027, 0.558±0.097, 0.561±0.029, and 0.638±0.015,respectively. From results for 28 M dwarfs analyzed homogeneously from APOGEEspectra, we find exoplanet-hosting M dwarfs exhibit a C/O abundance ratioapproximately 0.01 to 0.05 higher than those with non-detected exoplanets, atlimits of a statistically significant offset. A linear regression of [Fe/H]vs. C/O distribution reveals a noticeable difference in the angularcoefficient between FGK dwarfs (0.27) and M dwarfs (0.13). Assuming ourabundance ratios of Ca/Mg, Si/Mg, Al/Mg, and Fe/Mg, we determine a mass of3.276^+0.448_-0.419M_⊕ for TOI-2445 b, having density(6.793^+0.005_-0.099 g.cm^-3) and core mass fraction(0.329_-0.049^+0.028) very similar to Earth's. We also present an atlasof 113 well-defined spectral lines to analyze M dwarfs in the H-band and acomprehensive evaluation of uncertainties from variations in the atmosphericparameters, signal-to-noise, and pseudo-continuum.