  We determine the effective temperature, surface gravity and projected
rotational velocity of nine T dwarfs 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. We use the AMES-COND cloudless solar
metallicity models provided by the PHOENIX code to match the spectra of the T
dwarfs observed with the NIR high-resolution spectrograph NIRSPEC using ten
echelle orders to cover part of the J band from 1.165 to 1.323 microns with
R~20,000. The projected rotational velocity, Teff and logg of the objects are
determined based on the minimum RMS of the differences between the modelled and
observed relative fluxes. The modelled spectra reproduce quite well the
observed features for most of the T dwarfs, with Teff in the range of 922-1009
K, and logg between 4.3 and 5 (cm s^-2). Our results support the assumption of
a dust free atmosphere for T dwarfs 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 KI doublet at
1.2436 and 1.2525 microns. Our modelled spectra 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 in the
models used here. By comparing our results and their uncertainties to
evolutionary models, we estimate masses in the interval ~5-75 MJ for T dwarfs
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 few Gyr for
the same T dwarfs, which is consistent with recent kinematical studies.
