Variability of Southern T Tauri Stars. II. The Spectral Variability of the Classical T Tauri Star TW Hydrae

Abstract

We present the analysis of 42 spectra of the Classical T Tauri star TW Hya observed with the FEROS echelle spectrograph over 2 yr. We determined the rotational and radial velocities of TW Hya, obtaining v sin i = 5 ± 2 km s -1 and v rad = 12.5 ± 0.5 km s -1 . The star exhibits strong emission lines that show substantial variety and variability in their profile shapes. Emission lines such as Hα, Hβ, and He I show both outflow and infall signatures, which change on different timescales. The system displays periodic variations in line and veiling intensities, but the stellar rotation period remains uncertain. We see evidence of a variation in the mass accretion rate over a 1 yr period from the Na D line profiles that are well fitted by magnetospheric accretion models with moderate mass accretion rates (10 -9 up to 10 -8 M ⊙ yr -1 ). The lower values inferred from the models are close to the average mass accretion rate obtained from the veiling estimates (∼2 × 10 -9 M ⊙ yr -1 ), but the veiling results are consistent with a constant mass accretion rate within the errors of the calculations. The Ha, He I, Na D, and Hβ emission-line equivalent widths corrected from veiling correlate well with each other and are correlated with the veiling, indicating the same mechanism should be powering them and suggesting an origin related to the accretion process. The wings of the main emission lines are generally correlated, except when the Balmer lines exhibit properties suggesting a strong contribution from a wind. The blueward absorption components of the Balmer lines, most likely from a wind, are not correlated with veiling. The spectroscopic analysis allows us to infer the inclination of the stellar rotation axis (i = 18° ± 10°) that matches the current estimations of the disk orientation (0° < i < 15°). A magnetospheric dipole axis that is misaligned with the stellar/disk rotation axis could produce the observed photometric variability, and we tend to favor a low inclination but not a totally face-on geometry for the system. TW Hya exhibits typical spectral characteristics of many classical T Tauri stars in Taurus despite its older age, indicating that active accretion disks can readily survive up to 10 Myr.