An H-band Spectroscopic Metallicity Calibration for M Dwarfs

TL;DR

This paper develops an empirical H-band spectroscopic method to estimate M dwarf metallicities with high accuracy, useful for studying exoplanets and stellar compositions in large NIR surveys.

Contribution

It introduces the first empirically calibrated H-band metallicity estimator for M dwarfs, matching K-band methods and enabling improved metallicity measurements in NIR spectra.

Findings

H-band and K-band calibrations yield consistent metallicities with ±0.12 dex accuracy.

Confirmed super-solar metallicity of the planet-hosting M dwarf GJ 317.

M dwarfs with giant planets tend to be more metal-rich than those with smaller planets.

Abstract

We present an empirical near-infrared (NIR) spectroscopic method for estimating M dwarf metallicities, based on features in the H-band, as well as an implementation of a similar published method in the K-band. We obtained R~2000 NIR spectra of a sample of M dwarfs using the NASA IRTF-SpeX spectrograph, including 22 M dwarf metallicity calibration targets that have FGK companions with known metallicities. The H-band and K-band calibrations provide equivalent fits to the metallicities of these binaries, with an accuracy of +/- 0.12 dex. We derive the first empirically calibrated spectroscopic metallicity estimate for the giant planet-hosting M dwarf GJ 317, confirming its super-solar metallicity. Combining this result with observations of eight other M dwarf planet hosts, we find that M dwarfs with giant planets are preferentially metal-rich compared to those that host less massive planets. Our H-band calibration relies on strongly metallicity-dependent features in the H-band, which will be useful in compositional studies using mid to high resolution NIR M dwarf spectra, such as those produced by multiplexed surveys like SDSS-III APOGEE. These results will also be immediately useful for ongoing spectroscopic surveys of M dwarfs.