Spectropolarimetry of Magnetospheric Accretion on the Classical T Tauri Star BP Tauri

High-resolution ( R ≈ 60,000) circular spectropolarimetry of the classical T Tauri star BP Tauri is presented. No net polarization is detected in photospheric absorption lines, placing strong limits (3 σ) on the mean longitudinal magnetic field (±200 G) that is present over the surface of the star. On the other hand, strong circular polarization is measured in the He I λ5876 emission line of BP Tau, indicating a mean longitudinal magnetic field of 2460±120 G in the line formation region. This implies that accretion occurs preferentially along large-scale magnetic loops that occupy a small fraction of the stellar surface. These observations represent the first direct evidence for magnetically controlled accretion onto classical T Tauri stars.