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The discovery of a new spectral class – the Y dwarfs – by NASA's Wide-field Infrared Survey Explorer has enabled researchers to study the physics of the coldest brown dwarfs and to determine the prevalence of brown dwarfs in the Milky Way. The boundary between T dwarfs and Y dwarfs roughly coincides with the location where the J – H colors of brown dwarfs, as predicted by models, turn back to the red at effective temperatures below ∼400 K. Preliminary trigonometric parallax measurements show that the T/Y boundary appears also to correspond to the point at which the absolute H (1.6 µm) and W2 (4.6 µm) magnitudes plummet. Using these new distance measurements, it is found that hydrogenburning stars outnumber brown dwarfs by approximately a factor of six in the solar neighborhood. Using simulations describing the mass function as a power law or as a lognormal, we find that both the shape and predicted space densities in the mid-T to early-Y regime can be fit only if these functions are normalized downward to better reflect the relative paucity of brown dwarfs. The sample of currently identified Y dwarfs probes the field mass function to masses of ∼5 M_(Jup), but confirmation of additional, colder Y dwarfs from WISE is needed to determine whether the low-mass limit to star formation has yet been measured.

The launch of NASA’s Wide-field Infrared Survey Explorer (WISE) in late 2009 ushered in an new era in the study of the solar neighborhood. Its mid-infrared capabilities have proven critical to the discovery of the bulk of the coolest (T_(eff)