Properties of the T8.5 dwarf Wolf 940 B

We present 7.5-14.2 μm low-resolution spectroscopy, obtained with the Spitzer Infrared Spectrograph, of the T8.5 dwarf Wolf 940 B, which is a companion to an M4 dwarf with a projected separation of 400 AU. We combine these data with previously published near-infrared spectroscopy and mid-infrared photometry to produce the spectral energy distribution for the very low temperature T dwarf. We use atmospheric models to derive the bolometric correction and obtain a luminosity of log L/L_odot; = -6.01± 0.05 (the observed spectra make up 47% of the total flux). Evolutionary models are used with the luminosity to constrain the values of effective temperature (T_eff) and surface gravity and hence mass and age for the T dwarf. We ensure that the spectral models used to determine the bolometric correction have T_eff and gravity values consistent with the luminosity-implied values. We further restrict the allowed range of T_eff and gravity using age constraints implied by the M dwarf primary and refine the physical properties of the T dwarf by comparison of the observed and modeled spectroscopy and photometry. This comparison indicates that Wolf 940 B has a metallicity within ∼0.2 dex of solar, as more extreme values give poor fits to the data-lower metallicity produces a poor fit at λ > 2 μm, while higher metallicity produces a poor fit at λ < 2 μm. This is consistent with the independently derived value of [m/H] = +0.24 ± 0.09 for the primary star, using the Johnson & Apps MK: V - K relationship. We find that the T dwarf atmosphere is undergoing vigorous mixing, with an eddy diffusion coefficient K_zz of 10⁴ to 10⁶ cm ² s^-1. We derive an effective temperature of 585 K to 625 K, and surface gravity log g = 4.83 to 5.22 (cm s-^-2), for an age range of 3 Gyr to 10 Gyr, as implied by the kinematic and Ha properties of the M dwarf primary. Gravity and temperature are correlated such that the lower gravity corresponds to the lower temperature and younger age for the system and the higher values to the higher temperature and older age. The mass of the T dwarf is 24 M_jupiter to 45 M_Jupiter for the younger to older age limit.