We present near-infrared photometry and spectroscopy , and warm- Spitzer IRAC photometry of the young very cool T dwarf Ross 458C , which we have typed as T8.5p .
By applying the fiducial age constraints ( \leq 1 Gyr ) imposed by the properties of the active M dwarf Ross 458A , we have used these data to determine that Ross 458C has T _ { eff } = 695 \pm 60 K , \log g = 4.0 - 4.7 and an inferred mass of 5–20 M _ { Jup } .
We have compared fits of the near-infrared spectrum and IRAC photometry to the BT Settl and Saumon & Marley model grids , and have found that both sets provide best fits that are consistent with our derived properties , whilst the former provide a marginally closer match to the data for all scenarios explored here .
The main difference between the model grids arises in the 4.5 \micron region , where the BT Settl models are able to better predict the flux through the IRAC filter , suggesting that non-equilibrium effects on the CO-CO _ { 2 } ratio are important for shaping the mid-infrared spectra of very cool T dwarfs .
We have also revisited the issue of dust opacity in the spectra of Ross 458C that was raised by Burgasser et al ( 2010 ) .
We have found that the BT Settl models which also incorporate a condensate cloud model , provide a better match to the near-infrared spectrum of this target than the Saumon & Marley model with f _ { sed } = 2 , and we briefly discuss the influence of condensate clouds on T dwarf spectra .