WISE J163940.83–684738.6: A Y Dwarf Identified by Methane Imaging

We have used methane imaging techniques to identify the near-infrared counterpart of the bright Wide-field Infrared Survey Explorer (WISE) source WISE J163940.83–684738.6. The large proper motion of this source (≈3."0 yr^(–1)) has moved it, since its original WISE identification, very close to a much brighter background star—it currently lies within 1farcs5 of the J = 14.90 ± 0.04 star 2MASS 16394085–6847446. Observations in good seeing conditions using methane-sensitive filters in the near-infrared J band with the FourStar instrument on the Magellan 6.5 m Baade telescope, however, have enabled us to detect a near-infrared counterpart. We have defined a photometric system for use with the FourStar J2 and J3 filters, and this photometry indicates strong methane absorption, which unequivocally identifies it as the source of the WISE flux. Using these imaging observations we were then able to steer this object down the slit of the Folded-port Infrared Echellette spectrograph on a night of 0."6 seeing, and so obtain near-infrared spectroscopy confirming a Y0-Y0.5 spectral type. This is in line with the object's near-infrared-to-WISE J3 – W2 color. Preliminary astrometry using both WISE and FourStar data indicates a distance of 5.0 ± 0.5 pc and a substantial tangential velocity of 73 ± 8 km s^(–1). WISE J163940.83–684738.6 is the brightest confirmed Y dwarf in the WISE W2 passband and its distance measurement places it among the lowest luminosity sources detected to date