NEAR-INFRARED SPECTROSCOPY OF TW Hya: A REVISED SPECTRAL TYPE AND COMPARISON WITH MAGNETOSPHERIC ACCRETION MODELS

We present high signal-to-noise ratio (S/N), moderate spectral resolution (R similar to 2000-2500) near-infrared (0.8-5.0 mu m) 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 what is 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 similar to 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) similar to 10(12)-10(13) cm(-3). The thickness of the emitting region is 10(2)-10(4) km and the covering fraction is f(*) similar to 0.04. Our derived physical parameter values agree with the predictions of the magnetospheric accretion scenario. The highest S/N 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 (similar to 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.