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

Abstract Exoplanets hosting M dwarfs are the best targets to characterize Earth-like or super-Earth planetary atmospheres with the James Webb Space Telescope (JWST). We determine detailed stellar parameters ( T eff , log g , and ξ ) and individual abundances of 12 elements for four cool M dwarfs hosting exoplanets TOI-1685, GJ 436, GJ 3470, and TOI-2445, scheduled for future observations by the JWST. The analysis utilizes high-resolution near-infrared spectra from the Sloan Digital Sky Survey IV APOGEE survey between 1.51 and 1.69 μ m. Based on 1D LTE plane-parallel models, we find that TOI-2445 is slightly metal poor ([Fe/H] = −0.16 ± 0.09 dex), while TOI-1685, GJ 436, and GJ 3470 are more metal rich ([Fe/H] = 0.06 ± 0.18, 0.10 ± 0.20, and 0.25 ± 0.15 dex, respectively). The derived C/O ratios for TOI-2445, TOI-1685, GJ 436, and GJ 3470 are 0.526 ± 0.027, 0.558 ± 0.097, 0.561 ± 0.029, and 0.638 ± 0.015, respectively. From the results for 28 M dwarfs analyzed homogeneously from Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectra, we find exoplanet-hosting M dwarfs exhibit a C/O abundance ratio approximately 0.01–0.05 higher than those with nondetected exoplanets, at limits of a statistically significant offset. A linear regression of the [Fe/H] versus C/O distribution reveals a noticeable difference in the angular coefficient between FGK dwarfs (0.27) and M dwarfs (0.13). Assuming our abundance ratios of Ca/Mg, Si/Mg, Al/Mg, and Fe/Mg, we determine a mass of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>3.276</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.419</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.448</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> M ⊕ for TOI-2445 b, which has a density (6.793 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.099</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.005</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> g cm −3 ) and core mass fraction (0.329 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.049</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.028</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ) very similar to Earth’s. We also present an atlas of 113 well-defined spectral lines to analyze M dwarfs in the H band and a comprehensive evaluation of uncertainties from variations in the atmospheric parameters, signal-to-noise ratio, and pseudocontinuum.