DISCOVERY OF THE Y1 DWARF WISE J064723.23−623235.5

Abstract

We present the discovery of a very cold, very low mass, nearby brown dwarf using data from the NASA Wide-field Infrared Survey Explorer (WISE). The object,WISE J064723.23−623235.5, has a very red WISE color ofW1−W2> 3.77 mag and a very red Spitzer Space Telescope color of ch1−ch2 = 2.82 ± 0.09 mag. In JMKO−ch2 color (7.58 ± 0.27 mag) it is one of the two or three reddest brown dwarfs known. Our grism spectrum from the Hubble Space Telescope (HST) confirms it to be the seventeenth Y dwarf discovered, and its spectral type of Y1 ± 0.5 makes it one of the four latest-type Y dwarfs classified. Astrometric imaging from Spitzer and HST, combined with data from WISE, provides a preliminary parallax of π = 115 ± 12 mas (d = 8.7 ± 0.9 pc) and proper motion of μ = 387 ± 25 mas yr−1 based on 2.5 yr of monitoring. The spectrum implies a blue J−H color, for which model atmosphere calculations suggest a relatively low surface gravity. The best fit to these models indicates an effective temperature of 350–400 K and a mass of ∼5–30 MJup. Kinematic analysis hints that this object may belong to the Columba moving group, which would support an age of ∼30 Myr and thus an even lower mass of <2 MJup, but verification would require a radial velocity measurement not currently possible for a J = 22.7 mag brown dwarf.