WISE J163940.83−684738.6 (designation is abbreviated to WISE 1639−6847, or W1639) is a brown dwarf of spectral class Y0 pec, located in constellation Triangulum Australe (it's the nearest star / brown dwarf in this constellation) at 15.5 light-years from Earth. It is the second closest Y-dwarf as of February 2024.
WISE 1639−6847 was discovered in 2012 by C. G. Tinney et al. from data, collected by Wide-field Infrared Survey Explorer (WISE) Earth-orbiting satellite — NASA infrared-wavelength 40 cm (16 in) space telescope, which mission lasted from December 2009 to February 2011.
In 2012 Tinney et al. carried out follow-up observations of WISE 1639−6847 using the FourStar infrared mosaic camera mounted on the 6.5 m Magellan Baade telescope at Las Campanas Observatory, Chile (on 2012 May 10–11 (UT)); and spectroscopy using the Folded-port Infrared Echellette (FIRE) also mounted on the 6.5 m Magellan Baade telescope (on 2012 July 10 (UT)).
In 2012 Tinney et al. published a paper in The Astrophysical Journal, where they presented discovery of a newfound by WISE Y-type brown dwarf WISE 1639−6847 (the only brown dwarf discovery, presented in the article): the paper was accepted for publication on 20 September 2012, submitted to arXiv on 27 September 2012, and published in November 2012.
WISE 1639−6847 has absolute magnitude in J-band 22.14 ± 0.22. WISE 1639−6847 was first classified to have a spectral type between Y0 and Y0.5. Observations with Hubble WFC3 near-infrared grism spectroscopy showed that the J-band peak matched with the Y0 standard. The Y-band peak and the Y-J color showed that it was unusual compared to other Y-dwarfs and therefore a spectral type of Y0 pec was applied. The pec stands for peculiar or unusual. Modelling of this Y-dwarf struggles to reproduce the spectrum. Only with rather unrealistic high temperature and low gravity it was possible to reproduce the spectrum.