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Knowledge of late K and M dwarf metallicities can be used to guide planetsearches and constrain planet formation models. However, the determination ofmetallicities of late-type stars is difficult because visible wavelengthspectra of their cool atmospheres contain many overlapping absorption lines,preventing the measurement of equivalent widths. We present new methods, andimproved calibrations of existing methods, to determine metallicities of late-Kand M dwarfs from moderate resolution (1300 < R < 2000) visible and infraredspectra. We select a sample of 112 wide binary systems that contain a late-typecompanion to a solar-type primary star. Our sample includes 62 primary starswith previously published metallicities, as well as 50 stars with metallicitiesdetermined from our own observations. We use our sample to empiricallydetermine which features in the spectrum of the companion are best correlatedwith the metallicity of the primary. We derive metallicity calibrations fordifferent wavelength ranges, and show that it is possible to get metallicitiesreliable to < 0.10 dex using either visible, J, H, or K band spectra. Ourcalibrations 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 widebinaries to test and refine existing calibrations to determine M dwarfmetallicities. We find that the zeta parameter, which measures the ratio of TiOcan CaH bands, is correlated with [Fe/H] for super-solar metallicities, andzeta does not always correctly identify metal-poor M dwarfs. We also find thatexisting calibrations in the K and H band are quite reliable for stars with[Fe/H] > -0.5, but are less useful for more metal-poor stars.

Prospecting in late-type dwarfs: a calibration of infrared and visible spectroscopic metallicities of late-K and M dwarfs spanning 1.5 dex