1 MILLIMETER POLARIMETRY OF YOUNG STELLAR OBJECTS: LOW-MASS PROTOSTARS AND T TAURI STARS

摘要

We have measured the polarization of the 1.1 mm and 0.8 mm continuum emission for three pre-T Tauri stars and two T Tauri stars. Positive detections were made for NGC 1333 IRAS 4A and IRAS 16293-2422, while L1551 IRS 5 and HL Tau were only marginally detected. For GG Tau we measured a 2 a upper limit of 3%. The polarization is interpreted in terms of thermal emission by magnetically aligned dust grains in circumstellar disks or envelopes. In the case of pre-T Tauri stars, the emission is most likely due to the dust in a large-scale (> 1000 AU) envelope with a disklike shape. The magnetic field direction inferred from our polarization at millimeter wavelengths is perpendicular to the major axis of the envelope and tends to be parallel to the embedding cloud magnetic field. The geometry suggests that the envelope has contracted along the magnetic field, resulting in a shape elongated perpendicular to the magnetic field. In contrast, the millimeter-wavelength emission of the T Tauri star, GG Tau, appears to originate from a small-scale (< 100 AU) accretion disk, which is rather common to T Tauri stars. Nondetection of millimeter wavelength polarization for GG Tau suggests either that there is no strong magnetic field in the disk or that the dust grains are not well aligned in the disk. Marginal polarization of HL Tau is also discussed in the context of an evolution of the protostellar environment.