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 Tdwarfs from the comparison of high-resolution near-infrared spectra and synthetic models, and estimate the mass and age of the objects from state-of-the-art models. Methods. We use the AMES-COND cloudless solar metallicity models provided by the PHOENIX code to match the spectra of nine T-type field dwarfs observed with the near-infrared high-resolution spectrograph NIRSPEC using ten echelle orders to cover part of the J band from 1.147 to 1.347m with a resolving power .The projected rotational velocity, effective temperature and surface gravity of the objects are determined based on the minimum root mean square of the differences between the modelled and observed relative fluxes. Estimates of the mass and age of the objects are obtained from effective temperature-surface gravity diagrams, where our results are compared with existing solar metallicity models. Results. The modelled spectra reproduce quite well the observed features for most of the Tdwarfs, with effective temperatures in the range of 922-1009K, and surface gravities between 104.1 and 104.9cms-2. Our results support the assumption of a dust free atmosphere for Tdwarfs later than T5, where dust grains form and then gravitationally sediment into the low atmosphere. The modelled spectra do not accurately mimic some individual very strong lines like the K I doublet at 1.2436 and 1.2525m. Our modelled spectra does not match well the observed spectra of the two Tdwarfs with earlier spectral types, namely SDSSpJ125453.90-012247.4 (T2) and 2MASS J05591914-1404488 (T4.5), which is likely due to the presence of condensate clouds that are not incorporated in the models used here. By comparing our results and their uncertainties to evolutionary models, we estimate masses in the interval for Tdwarfs later than T5, which are in good agreement with those found in the literature. We found apparent young ages that are typically between 0.1 and a fewGyr for the same Tdwarfs, which is consistent with recent kinematical studies. Key words: stars: fundamental parameters - stars: atmosphers - stars: low-mass, brown dwarfs