Infrared Observations and Modeling of One of the Coolest T Dwarfs: Gliese 570D

We have obtained a good-quality R~400, 0.8-2.5 μm spectrum as well as accurate photometry of Gliese 570D, one of the coolest and least-luminous brown dwarfs currently known. The spectrum shows that Gl 570D has deeper absorptions in the strong water and methane bands at 1.12-1.17, 1.33-1.45, 1.62-1.88, and 2.20-2.45 μm and is both bluer at J-K and redder at K-L' than previously observed T dwarfs. Data analysis using model spectra coupled with knowledge of the well-understood primary implies that for the same surface gravity, Gl 570D is about 160 K cooler than Gl 229B. For an age range of 2-5 Gyr, Gl 570D has an effective temperature in the range 784-824 K, a surface gravity logg in the range 5.00-5.27 (cm s^-2), and a luminosity in the range (2.88-2.98)×10^-6 L_solar.