Discovery of a brown dwarf companion to gliese 570ABC: A 2mass T dwarf significantly cooler than gliese 229B

We report the discovery of a widely separated (258." 3 + 0." 4) T dwarf com panion to the Gl 570ABC system. This new component, Gl 570D, was initially identified from the Two Micron All-Sky Survey. Its near-infrared spectrum s hows the 1.6 and 2.2 mu m CH4 absorption bands characteristic of T dwarfs, while its common proper motion with the Gl 570ABC system confirms companion ship. Gl 570D (M-j = 16.47 +/- 0.07) is nearly a full magnitude dimmer than the only other known T dwarf companion, Gl 229B, and estimates of L = (2.8 +/- 0.3) x 10(-6) L. and T-eff = 750 +/- 50 K make it significantly cooler and less luminous than any other known brown dwarf companion. Using evolut ionary models by Burrows et al. and an adopted age of 2-10 Gyr, we derive a mass estimate of 50 +/- 20 M-Jup for this object.