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

Abstract

High-resolution (R ¼ 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 j) on the mean longitudinal magnetic field (5200 G) that is present over the surface of the star. On the other hand, strong circular polarization is measured in the He i l5876 emission line of BP Tau, indicating a mean longitudinal magnetic field of 24605120 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.