Near-Infrared Spectroscopy of the Cool Brown Dwarf, SDSS 1624+00

Using the Subaru Telescope, we have obtained multiple near-infrared spectra of the cool brown dwarf, SDSS 1624 + 00 (J162414.37+002915.8), in search of spectral variability in an 80 minute time span. We have found the suspected variability of water vapor absorption throughout the observations, which requires a confirmation with a longer time baseline. After coadding the spectra, we have obtained a high-quality spectrum covering from 1.05 to 1.8 /im. There are three kinds of spectral indicators, the water vapor bands, methane band and Kl lines at 1.243 and 1.252 //m, which can be used to study the temperature and the presence of dust. We compare the spectra of SDSS 1624+00 and Gliese 229B, while paying special attention to these indicators. The shallower water vapor absorption of SDSS 1624+00 indicates that it is warmer and/or dustier. The shallower methane absorption suggests that SDSS 1624+00 is warmer. We interpret the deeper KI lines in SDSS 1624+00 as being the result of its higher temperature. With the help of model spectra, we conclude that SDSS 1624+00 is warmer and dustier than Gliese 229B. For the first time in a cool brown dwarf, a finite flux is seen at the bottom of the water vapor band between 1.34 and 1.42 /im, which means that the 1.4 /xm band of water can be completely observed from the ground.