Knowledge of late K and M dwarf metallicities can be used to guide planet searches and constrain planet formation models .
However , the determination of metallicities of late-type stars is difficult because visible wavelength spectra of their cool atmospheres contain many overlapping absorption lines , preventing the measurement of equivalent widths .
We present new methods , and improved calibrations of existing methods , to determine metallicities of late-K and M dwarfs from moderate resolution ( 1300 < R < 2000 ) visible and infrared spectra .
We select a sample of 112 wide binary systems that contain a late-type companion to a solar-type primary star .
Our sample includes 62 primary stars with previously published metallicities , as well as 50 stars with metallicities determined from our own observations .
We use our sample to empirically determine which features in the spectrum of the companion are best correlated with the metallicity of the primary .
We find \simeq 120 features in K and M dwarf spectra that are useful for predicting metallicity .
We derive metallicity calibrations for different wavelength ranges , and show that it is possible to get metallicities reliable to \lesssim 0.10 Â dex using either visible , J - , H - , or K - band spectra .
We find that the most accurate metallicities derived from visible spectra requires the use of different calibrations for early-type ( K5.5–M2 ) and late-type ( M2–M6 ) dwarfs .
Our calibrations are applicable to dwarfs with metallicities of -1.04 < [ Fe/H ] < +0.56 and spectral types from K7 to M5 .
Lastly , we use our sample of wide binaries to test and refine existing calibrations to determine M dwarf metallicities .
We find that the \zeta parameter , which measures the ratio of TiO can CaH bands , is correlated with [ Fe/H ] for super-solar metallicities , and \zeta does not always correctly identify metal-poor M dwarfs .
We also find that existing calibrations in the K and H bands are quite reliable for stars with [ Fe/H ] > -0.5 , but are less useful for more metal-poor stars .