A method for selecting M dwarfs with an increased likelihood of unresolved ultracool companionship

Locating ultracool companions to M dwarfs is important for constraining low-mass formation models, the measurement of substellar dynamical masses and radii, and for testing ultracool evolutionary models. We present an optimized method for identifying M dwarfs which may have unresolved ultracool companions. We construct a catalogue of 440 694 M dwarf candidates, from Wide-Field Infrared Survey Explorer, Two Micron All-Sky Survey and Sloan Digital Sky Survey, based on optical- and near-infrared colours and reduced proper motion. With strict reddening, photometric and quality constraints we isolate a subsample of 36 898 M dwarfs and search for possible mid-infrared M dwarf + ultracool dwarf candidates by comparing M dwarfs which have similar optical/near-infrared colours (chosen for their sensitivity to effective temperature and metallicity). We present 1082 M dwarf + ultracool dwarf candidates for follow-up. Using simulated ultracool dwarf companions to M dwarfs, we estimate that the occurrence of unresolved ultracool companions amongst our M dwarf + ultracool dwarf candidates should be at least four times the average for our full M dwarf catalogue. We discuss possible contamination and bias and predict yields of candidates based on our simulations.