We present a Hubble Space Telescope/Wide-Field Camera 3 near infrared spectrum of the archetype Y dwarf WISEP 182831.08+265037.8. The spectrum covers the 0.9-1.7 um wavelength range at a resolving power of lambda/Delta lambda ~180 and is a significant improvement over the previously published spectrum because it covers a broader wavelength range and is uncontaminated by light from a background star. The spectrum is unique for a cool brown dwarf in that the flux peaks in the Y, J, and H band are of near equal intensity in units of f_lambda. We fail to detect any absorption bands of NH_3 in the spectrum, in contrast to the predictions of chemical equilibrium models, but tentatively identify CH_4 as the carrier of an unknown absorption feature centered at 1.015 um. Using previously published ground- and spaced-based photometry, and using a Rayleigh Jeans tail to account for flux emerging longward of 4.5 um, we compute a bolometric luminosity of log (L_bol/L_sun)=-6.50+-0.02 which is significantly lower than previously published results. Finally, we compare the spectrum and photometry to two sets of atmospheric models and find that best overall match to the observed properties of WISEP 182831.08+265037.8 is a ~1 Gyr old binary composed of two T_eff~325 K, ~5 M_Jup brown dwarfs with subsolar [C/O] ratios.
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