We present high signal-to-noise, moderate spectral resolution (R ~ 2000-2500)
near-infrared (0.8-5.0 micron) spectroscopy of the nearby T Tauri star TW Hya.
By comparing the spectrum and the equivalent widths of several atomic and
molecular features with those for stars in the IRTF near-infrared library, we
revise the spectral type to M2.5V, which is later than usually adopted (K7V).
This implies a substantially cooler stellar temperature than previously
assumed. Comparison with various pre-main sequence models suggests that TW Hya
is only ~3 Myr old; much younger than the usually adopted 8 - 10 Myr. Analysis
of the relative strengths of the H lines seen in the spectrum yields estimates
for the temperature and density of the emitting region of T_e > 7500 K and n_e
~ 10^{12} - 10^{13} cm^{-3}. The thickness of the emitting region is 10^2 -
10^4 km and the covering fraction is f_\ast ~ 0.04. Our derived physical
parameter values agree with the predictions of the magnetospheric accretion
scenario. The highest signal-to-noise H lines have profiles that indicate
multiple emission components. We derive an excess spectrum (above that of the
M2.5V template) that peaks in the H band. Although our derived veiling values,
~ 0.1, agree with previous estimates, the excess spectrum does not match that
of current models in which this flux is generated by an inner optically thin
disk. We suggest that the excess flux spectrum instead reflects the differences
in atmospheric opacity, gravity, and age between TW Hya and older, higher
gravity field M2.5 dwarfs.Comment: 43 pages, 12 figures; accepted for publication in Ap