Near-infrared spectroscopy of the cool brown dwarf, SDSS 1624+00

Abstract

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 μm. There are three kinds of spectral indicators, the water vapor bands, methane band and KI lines at 1.243 and 1.252 μm, 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 μm, which means that the 1.4 μm band of water can be completely observed from the ground.