Near-Infrared Spectroscopy of TW Hya: A Revised Spectral Type and Comparison with Magnetospheric Accretion Models

TL;DR

This study uses near-infrared spectroscopy to revise TW Hya's spectral type to M2.5V, estimate its age at about 3 million years, and compare observed accretion features with magnetospheric models, revealing complex emission structures.

Contribution

It provides a revised spectral classification, age estimate, and physical parameters of TW Hya, and compares observed accretion signatures with theoretical models, highlighting discrepancies in excess flux origins.

Findings

TW Hya's spectral type is revised to M2.5V.

Estimated age of TW Hya is approximately 3 million years.

Observed emission line profiles suggest multiple components.

Abstract

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.