Physical parameters of T dwarfs derived from high-resolution near-infrared spectra

摘要

Aims. We determine the effective temperature, surface gravity and projected rotational velocity of nine T dwarfs from the compari-son of high-resolution near-infrared spectra and synthetic models, and estimate the mass and age of the objects from state-of-the-artmodels.Methods. We use the AMES-COND cloudless solar metallicity models provided by the PHOENIX code to match the spectra of nineT-type field dwarfs observed with the near-infrared high-resolution spectrograph NIRSPEC using ten echelle orders to cover partof the J band from 1.147 to 1.347 μm with a resolving power R ~ 20000. The projected rotational velocity, effective temperatureand surface gravity of the objects are determined based on the minimum root mean square of the differences between the modelledand observed relative fluxes. Estimates of the mass and age of the objects are obtained from effective temperature-surface gravitydiagrams, where our results are compared with existing solar metallicity models.Results. The modelled spectra reproduce quite well the observed features for most of the T dwarfs, with effective temperatures inthe range of 922-1009 K, and surface gravities between 10~(4.1)and 10~(4.9) cm s~(-2).Our results support the assumption of a dust freeatmosphere for T dwarfs later than T5, where dust grains form and then gravitationally sediment into the low atmosphere. The mod-elled spectra do not accurately mimic some individual very strong lines like the K I doublet at 1.2436 and 1.2525 pm. Our modelledspectra does not match well the observed spectra of the two T dwarfs with earlier spectral types, namely SDSSp J125453.90-012247.4(T2) and 2MASS J05591914-1404488 (T4.5), which is likely due to the presence of condensate clouds that are not incorporated inthe models used here. By comparing our results and their uncertainties to evolutionary models, we estimate masses in the interval≈5-75 M_J for T dwarfs later than T5, which are in good agreement with those found the literature. We apparent young agesthat are typically between 0.1 and a few Gyr for the same T dwarfs, which is consistent with recent kinematical studies.