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

We have measured the polarization of the 1.1 mm and 0.8 mm continuum e mission for three pre-T Tauri stars and two T Tauri stars. Positive de tections were made for NGC 1333 IRAS 4A and IRAS 16293-2422, while L15 51 IRS 5 and HL Tau were only marginally detected. For GG Tau we measu red a 2 sigma upper limit of 3%. The polarization is interpreted in te rms of thermal emission by magnetically aligned dust grains in circums tellar disks or envelopes. In the case of pre-T Tauri stars, the emiss ion is most likely due to the dust in a large-scale (>1000 AU) envelop e with a disklike shape. The magnetic held 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 m agnetic field. The geometry suggests that the envelope has contracted along the magnetic held, resulting in a shape elongated perpendicular to the magnetic held. In contrast, the millimeter-wavelength emission of the T Tauri star, GC Tan, appears to originate from a small-scale ( <100 AU) accretion disk, which is rather common to T Tauri stars. Nond etection of millimeter wavelength polarization for GG Tau suggests eit her that there is no strong magnetic field in the disk or that the dus t grains are not well aligned in the disk. Marginal polarization of HL Tau is also discussed in the context of an evolution of the protostel lar environment.