The properties of the T8.5p dwarf Ross 458C

Abstract

The definitive version can be found at: http://onlinelibrary.wiley.com/ Copyright The Royal Astronomical SocietyWe 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 (<= 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 +/- 60 K, log g = 4.0-4.7 and an inferred mass of 5-20M(J). 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-mu m 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-CO2 ratio are important for shaping the mid-infrared spectra of very cool T dwarfs. We have also revisited the issue of the dust opacity in the spectra of Ross 458C that was raised by Burgasser et al. 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

CFBDSIR2149-0403: a 4–7 Jupiter-mass free-floating planet in the young moving group AB Doradus?

“…This makes the direct comparison of these gravity-sensitive features difficult: they are slightly stronger in CFBDSIR2149 but this difference, including the presence of the 1.25 μm potassium doublet, might be fully accounted for by the difference in effective temperature rather than by an even lower gravity. To compare the colours of both objects using the same photometric system, we calculated the NIR colours of Ross458C in the WIRCam filter system using the spectra provided by Burningham et al (2011a). The resulting colours are shown in Table 5 and are very similar to those observed for CFBDSIR2149.…”

“…As discussed in Burgasser et al (2010) and Burningham et al (2011a), this object shows many features of low gravity, and it belongs to a young stellar system (150−800 Myr), with a metallicity that is slightly higher than solar ([Fe/H] ∼ 0.2−0.3).…”

“…9, for the same gravity and temperature but with a different cloud parameter setting), that one of the cloud settings matches the observed spectrum at some point on the gravity/temperature grid as well the BT-Settl or BSL2003 models do is not surprising. Burningham et al (2011a) also concludes that such adaptable cloud models were not necessary to account for Ross458C's spectrum, which can be fitted with BT-Settl models that use a self-consistent cloud model over their full parameter range.…”

“…Another interesting object to compare with is the very red T8.5 brown dwarf Ross458C (Scholz 2010;Goldman et al 2010;Burgasser et al 2010;Burningham et al 2011a), which …”

Atmospheres From Very Low-Mass Stars to Extrasolar Planets

“…Similarly, ammonia has been shown to be under-abundant (Saumon et al 2006). More recently, carbon dioxide, which under chemical equilibrium conditions is expected to form in only very small quantities in hydrogen-rich atmospheres, has been directly detected spectroscopically (Tsuji et al 2011) and inferred from mid-infrared photometry (Burningham et al 2011). This is understood as the result of the fact that the formation of methane and ammonia, which are expected to dominate the carbon and nitrogen chemistries, respectively, in the low temperature limit under equilibrium conditions (Lodders & Fegley 2006), has to compete against upmixing from the deeper and warmer atmospheric layers.…”