The discovery of a very cool, very nearby brown dwarf in the Galactic plane

We report the discovery of a very cool, isolated brown dwarf, UGPS 0722−05, with the United Kingdom Infrared Telescope Deep Sky Survey (UKIDSS) Galactic Plane Survey. The near-infrared spectrum displays deeper H2O and CH4 troughs than the coolest known T dwarfs and an unidentified absorption feature at 1.275 μm. We provisionally classify the object as a T10 dwarf but note that it may in future come to be regarded as the first example of a new spectral type. The distance is measured by trigonometric parallax as d= 4.1+0.6−0.5 pc, making it the closest known isolated brown dwarf. With the aid of Spitzer/Infrared Array Camera (IRAC) we measure H−[4.5]= 4.71. It is the coolest brown dwarf presently known – the only known T dwarf that is redder in H−[4.5] is the peculiar T7.5 dwarf SDSS J1416+13B, which is thought to be warmer and more luminous than UGPS 0722−05. Our measurement of the luminosity, aided by Gemini/T-ReCS N-band photometry, is L= 9.2 ± 3.1 × 10−7 L⊙. Using a comparison with well-studied T8.5 and T9 dwarfs we deduce Teff= 520 ± 40 K. This is supported by predictions of the Saumon & Marley models. With apparent magnitude J= 16.52, UGPS 0722−05 is the brightest of the ∼90 T dwarfs discovered by UKIDSS so far. It offers opportunities for future study via high-resolution near-infrared spectroscopy and spectroscopy in the thermal infrared.