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We present the discovery of seven ultracool brown dwarfs identified with the Wide-field Infrared Survey Explorer (WISE). Near-infrared spectroscopy reveals deep absorption bands of H₂O and CH₄ that indicate all seven of the brown dwarfs have spectral types later than UGPS J072227. 51-054031. 2, the latest type T dwarf currently known. The spectrum of WISEP J182831. 08+265037. 8 is distinct in that the heights of the J- and H-band peaks are approximately equal in units of fₗambda, so we identify it as the archetypal member of the Y spectral class. The spectra of at least two of the other brown dwarfs exhibit absorption on the blue wing of the H-band peak that we tentatively ascribe to NH₃. These spectral morphological changes provide a clear transition between the T dwarfs and the Y dwarfs. In order to produce a smooth near-infrared spectral sequence across the T/Y dwarf transition, we have reclassified UGPS J0722-0540 as the T9 spectral standard and tentatively assign WISEP J173835. 52+273258. 9 as the Y0 spectral standard. In total, six of the seven new brown dwarfs are classified as Y dwarfs: four are classified as Y0, one is classified as Y0 (pec? ), and WISEP J1828+2650 is classified as >Y0. We have also compared the spectra to the model atmospheres of Marley and Saumon and infer that the brown dwarfs have effective temperatures ranging from 300 K to 500 K, making them the coldest spectroscopically confirmed brown dwarfs known to date.
Cushing et al. (Tue,) studied this question.