We use newly observed and published near-infrared spectra, together with synthetic spectra obtained from model atmospheres, to derive physical properties of three of the latest type T dwarfs. A new R ≈1700 spectrum of the T7.5 dwarf HD 3651B, together with existing data, allows a detailed comparison to the well-studied and very similar dwarf Gl 570D. We find that HD 3651B has both higher gravity and higher metallicity than Gl 570D, with best-fit atmospheric parameters of Teff = 820-830 K, log g = 5.4-5.5, [m/H] = +0.2, and Kzz = 104 cm2 s-1. Its age is 8-12 Gyr, and its implied mass is 60-70 MJ. We perform a similar analysis of the T8 and T7.5 dwarfs 2MASS J09393548-2448279 and 2MASS J11145133-2618235 using published data, comparing them to the well-studied T8, 2MASS J04151954-0935066. We find that these two dwarfs have effectively the same Teff as the reference dwarf, and similar or slightly higher gravities, but lower metallicities. The derived parameters are Teff = 725-775 K and [m/H] = -0.3; log g = 5.3 - 5.45 for 2MASS J09393548-2448279 and log g = 5.0 - 5.3 for 2MASS J11145133-261823. The age and mass are ∼ 10 Gyr and 60 MJ for 2MASS J09393548-2448279, and ∼5 Gyr and 40 M J for 2MASS J11145133-261823. A serious limitation to such analyses is the incompleteness of the line lists for transitions of CH4 and NH3 at λ ≤ 1.7 μm, which are also needed for synthesizing the spectrum of the later, cooler, Y type. Spectra of Saturn and Jupiter, and of laboratory CH4 and NH3 gas, suggest that NH3 features in the Y and J bands may be useful as indicators of the next spectral type, and not features in the H and K bands, as previously thought. However, until cooler objects are found, or the line lists improve, large uncertainties remain, as the abundance of NH3 is likely to be significantly below the chemical equilibrium value. Moreover, inclusion of laboratory NH3 opacities in our models predicts band shapes that are discrepant with existing data. It is possible that the T spectral class will have to be extended to temperatures around 400 K, when water clouds condense in the atmosphere and dramatically change the spectral energy distribution of the brown dwarf.
- Infrared: stars
- Stars: Low-mass, brown dwarfs
- Stars: individual (2MASS J04151954-0935066, 2MASS J09393548-2448279, 2MASS J11145133-2618235, HD 3651B, Gl 570D)
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science