The MEarth Project is a photometric survey systematically searching the smallest stars nearest to the Sun for transiting rocky planets .
Since 2008 , MEarth has taken approximately two million images of 1844 stars suspected to be mid-to-late M dwarfs .
We have augmented this survey by taking nightly exposures of photometric standard stars and have utilized this data to photometrically calibrate the MEarth system , identify photometric nights , and obtain an optical magnitude with 1.5 \% precision for each M dwarf system .
Each optical magnitude is an average over many years of data , and therefore should be largely immune to stellar variability and flaring .
We combine this with trigonometric distance measurements , spectroscopic metallicity measurements , and 2MASS infrared magnitude measurements in order to derive a color-magnitude-metallicity relation across the mid-to-late M dwarf spectral sequence that can reproduce spectroscopic metallicity determinations to a precision of 0.1 dex .
We release optical magnitudes and metallicity estimates for 1567 M dwarfs , many of which did not have an accurate determination of either prior to this work .
For an additional 277 stars without a trigonometric parallax , we provide an estimate of the distance assuming solar neighborhood metallicity .
We find that the median metallicity for a volume limited sample of stars within 20 parsecs of the Sun is [ Fe/H ] = -0.03 \pm 0.008 , and that 29 / 565 of these stars have a metallicity of [ Fe/H ] = -0.5 or lower , similar to the low-metallicity distribution of nearby G-dwarfs .
When combined with the results of ongoing and future planet surveys targeting these objects , the metallicity estimates presented here will be important in assessing the significance of any putative planet-metallicity correlation .