We present metallicities , radial velocities and near-infrared spectral types for 447Â M dwarfs determined from moderate resolution ( R \approx 2000 ) near-infrared ( NIR ) spectra obtained with IRTF/SpeX .
These M dwarfs are primarily 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 for each star and new spectral templates for IRTF in the Y , J , H Â 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 27Â 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 36Â 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 2.2 \micron as the sole indicator for metallicity .
It has an accuracy of 0.12 \mathrm { dex } 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 < \mathrm { [ Fe / H ] } < +0.35 \mathrm { dex } .
We present a new color-color 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 2.2 \micron .
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 \mathrm { km } \mathrm { s } ^ { -1 } accuracy .