Variability of Southern T Tauri Stars (VASTT). III. The Continuum Flux Changes of the TW Hydrae Bright Spot*

摘要

We have collected low-resolution spectrophotometric data of the classical T Tauri star TW Hya in an effort to detect and to follow the excess continuum emission (veiling) and the line changes at λ 5100 ?. The deveiled and calibrated flux distribution resembles that of a 30 Myr K7-M1 star of radius R = 0.8 R☉, mass M = 0.7 M☉, and log g = 4.5. The anticorrelation between the veiling (in the B band) and the observed Balmer jump found by previous authors, based on large samples of classical T Tauri stars, is confirmed in TW Hya. The line emission luminosities of the H, Ca?II, and He?I lines correlate with one another throughout the series, supporting the claims that the bulk of the line emission is formed in a single region or that their growth is controlled by a common mechanism. Surprisingly, the line emission fluxes do not correlate with the veiling at 4250 ? (B band). The line luminosities are, in general, less than 1% of the continuum luminosities. The veiling time series presents a cyclic behavior at 4.4 ± 0.4 days. We collect all of the archival photometric data and analyze the B-band observations using different algorithms. We found solutions at either the 4.4 day timescale or one-half of this value. The data sets presenting the 2.2 day periodicity yield double-peaked light curves when folded at the 4.4 day timescale. We interpret the 4.4 day solution as the rotation period of the star. The veiling and the line emission measurements yield accretion luminosities for the series. We model the impacted area in the photosphere by an isothermal gas of a given density, temperature, and size (δ) whose parameters change as the star rotates. Estimates of the total spot area (δ), as a percentage of the stellar projected area, lie within the range 2.5 δ 6.0. The accretion luminosity of the impacted region does not remain constant throughout the series. The mass accretion rate (acc) that governs the luminosity varies within 1.0 × 10-9 M☉ yr-1 acc 4.8 × 10-8 M☉ yr-1. The spot luminosity and the associated acc are tightly correlated to the projected spot area, δ, and change their absolute value as the star spins. If most of the accretion is channeled to a single spot, its colatitude will be larger than 70°, indicating that the magnetic dipole is largely inclined.