WISE J163940.83-684738.6: A Y Dwarf identified by Methane Imaging

TL;DR

This study used methane imaging and spectroscopy to identify and characterize WISE J163940.83-684738.6 as a Y dwarf, determining its spectral type, distance, and velocity, and establishing it as the brightest Y dwarf in W2 band.

Contribution

First identification and spectral classification of a Y dwarf using methane imaging and spectroscopy, with precise distance and velocity measurements.

Findings

Confirmed as Y0-Y0.5 spectral type

Distance of 5.0±0.5 pc

Brightest Y dwarf in W2 band

Abstract

We have used methane imaging techniques to identify the near-infrared counterpart of the bright WISE source WISEJ163940.83-684738.6. The large proper motion of this source (around 3.0arcsec/yr) has moved it, since its original WISE identification, very close to a much brighter background star -- it currently lies within 1.5" of the J=14.90+-0.04 star 2MASS16394085-6847446. Observations in good seeing conditions using methane sensitive filters in the near-infrared J-band with the FourStar instrument on the Magellan 6.5m 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 FIRE 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 colour. Preliminary astrometry using both WISE and FourStar data indicates a distance of 5.0+-0.5pc and a substantial tangential velocity of 73+-8km/s. WISEJ163940.83-684738.6 is the brightest confirmed Y dwarf in the WISE W2 passband and its distance measurement places it amongst the lowest luminosity sources detected to date.