THE DISCOVERY OF Y DWARFS USING DATA FROM THE<i>WIDE-FIELD INFRARED SURVEY EXPLORER</i>(<i>WISE</i>)

Abstract

We present the discovery of seven ultracool brown dwarfs identified with the\nWide-field Infrared Survey Explorer (WISE). Near-infrared spectroscopy reveals\ndeep absorption bands of H_2O and CH_4 that indicate all seven of the brown\ndwarfs have spectral types later than UGPS J072227.51-054031.2, the latest type\nT dwarf currently known. The spectrum of WISEP J182831.08+265037.8 is distinct\nin that the heights of the J- and H-band peaks are approximately equal in units\nof f_lambda, so we identify it as the archetypal member of the Y spectral\nclass. The spectra of at least two of the other brown dwarfs exhibit absorption\non the blue wing of the H-band peak that we tentatively ascribe to NH_3. These\nspectral morphological changes provide a clear transition between the T dwarfs\nand the Y dwarfs. In order to produce a smooth near-infrared spectral sequence\nacross the T/Y dwarf transition, we have reclassified UGPS J0722-0540 as the T9\nspectral standard and tentatively assign WISEP J173835.52+273258.9 as the Y0\nspectral standard. In total, six of the seven new brown dwarfs are classified\nas Y dwarfs: four are classified as Y0, one is classified as Y0 (pec?), and\nWISEP J1828+2650 is classified as &gt;Y0. We have also compared the spectra to the\nmodel atmospheres of Marley and Saumon and infer that the brown dwarfs have\neffective temperatures ranging from 300 K to 500 K, making them the coldest\nspectroscopically confirmed brown dwarfs known to date.\n