We present 7.5–14.2 \mu m 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 \pm 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 modelled spectroscopy and photometry .
This comparison indicates that Wolf 940 B has a metallicity within \sim 0.2 dex of solar , as more extreme values give poor fits to the data — lower metallicity produces a poor fit at \lambda > 2 ~ { } \mu m while higher metallicity produces a poor fit at \lambda < 2 ~ { } \mu m. This is consistent with the independently derived value of [ m/H ] = +0.24 \pm 0.09 for the primary star , using the Johnson & Apps ( 2008 ) M _ { K } : V - K relationship .
We find that the T dwarf atmosphere is undergoing vigorous mixing , with an eddy diffusion coefficient K _ { zz } of 10 ^ { 4 } to 10 ^ { 6 } 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 H \alpha 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 .