THE COLDEST BROWN DWARF (OR FREE-FLOATING PLANET)?: THE Y DWARF WISE 1828+2650

We have monitored the position of the cool Y dwarf WISEPA J182831.08+265037.8 using a combination of ground- and space-based telescopes and have determined its distance to be 11.2{sup +1.3} {sub -1.0} pc. Its absolute H magnitude, M{sub H} = 22.21{sup +0.25} {sub -0.22} mag, suggests a mass in the range 0.5-20 M {sub Jup} for ages of 0.1-10 Gyr with an effective temperature in the range 250-400 K. The broad range in mass is due primarily to the unknown age of the object. Since the high tangential velocity of the object, 51 {+-} 5 km s{sup -1}, is characteristic of an old disk population, a plausible age range of 2-4 Gyr leads to a mass range of 3-6 M {sub Jup} based on fits to the (highly uncertain) COND evolutionary models. The range in temperature is due to the fact that no single model adequately represents the 1-5 {mu}m spectral energy distribution (SED) of the source, failing by factors of up to five at either the short or long wavelength portions of the SED. The appearance of this very cold object may be affected by non-equilibrium chemistry or low temperature condensates forming clouds, two atmospheric processes that are known tomore » be important in brown dwarf atmospheres but have proven difficult to model. Finally, we argue that there would have to be a very steep upturn in the number density of late-type Y-dwarfs to account for the putative population of objects suggested by recent microlensing observations. Whether WISE 1828+2650 sits at the low-mass end of the brown dwarf population or is the first example of a large number of 'free-floating' planets is not yet known.« less

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