Near-infrared Metallicities, Radial Velocities and Spectral Types for 447 Nearby M Dwarfs

TL;DR

This study provides a comprehensive set of near-infrared metallicities, spectral types, and radial velocities for 447 nearby M dwarfs, developing new calibration methods and templates to improve distance and metallicity estimates for these stars.

Contribution

It introduces new NIR spectral templates, calibrations for metallicity and distance, and demonstrates improved accuracy in radial velocity measurements for M dwarfs.

Findings

Metallicity calibration using the sodium doublet at 2.2um with 0.12 dex accuracy.

Distance calibration with 14% scatter based on NIR spectral type or continuum curvature.

Radial velocities measured with 4.4 km/s accuracy.

Abstract

We present metallicities, radial velocities and near-infrared spectral types for 447 M dwarfs determined from moderate resolution (R~2000) near-infrared (NIR) spectra obtained with IRTF/SpeX. These M dwarfs are targets of the MEarth Survey, a transiting planet survey searching for super Earths around mid-to-late M dwarfs within 33pc. We present NIR spectral types and new IRTF spectral templates in the Z, J, H and K-bands, created using M dwarfs with near-solar metallicities. We developed two spectroscopic distance calibrations that use NIR spectral type or an index based on the curvature of the K-band continuum. Our distance calibration has a scatter of 14%. We searched 27 NIR spectral lines and 10 spectral indices for metallicity sensitive features, taking into account correlated noise in our estimates of the errors on these parameters. We calibrated our relation using 36 M dwarfs in common proper pairs with an F, G or K-type star of known metallicity. We validated the physical association of these pairs using proper motions, radial velocities and spectroscopic distance estimates. Our resulting metallicity calibration uses the sodium doublet at 2.2um as the sole indicator for metallicity. It has an accuracy of 0.12dex inferred from the scatter between the metallicities of the primaries and the estimated metallicities of the secondaries. Our relation is valid for NIR spectral types from M1V to M5V and for -1.0<[Fe/H]<+0.35dex. We present a new metallicity relation using J-H and J-K colors that directly relates two observables: the distance from the M dwarf main sequence and equivalent width of the sodium line at 2.2um. We measured radial velocities by modeling telluric features to determine the absolute wavelength calibration of our spectra, and used M dwarf binaries, observations at different epochs, and comparison to precisely measured radial velocities to demonstrate 4.4km/s accuracy.