ALMA Observations of Polarized 872 μm Dust Emission from the Protostellar Systems VLA 1623 and L1527

Abstract

We present high-sensitivity (σ I ∼ 0.2-0.5 mJy, σ QU ∼ 0.05 mJy), high-resolution (∼0.″12 -0.″2) observations of polarized 872 μm dust emission from the young multiple system VLA 1623 in ρ Ophiuchus and the protostar L1527 in Taurus. We detect the circumstellar material of VLA 1623A, the extended Keplerian disk surrounding VLA 1623A that we call VLA 1623CBdisk, VLA 1623B, VLA 1623W, and L1527 strongly in the polarized emission, at the ∼1%-3% level. We spatially resolve VLA 1623A into two sources, VLA 1623Aa and VLA 1623Ab, separated by ∼30 au and located within a cavity of radius ∼50 au within the circumbinary Keplerian disk, as well as the edge-on disk of VLA 1623W. The polarization angle of the emission is uniform across each protostellar source and nearly coincides with each disk's minor axis. The offsets between the minor axis position angle and the polarization angle are not uniformly distributed at the P ≲ 2 ×10-4 level. The circumbinary disk surrounding VLA 1623Aab is azimuthally symmetrically polarized. Each compact source's emission is partially optically thick (τ 1) at 872 μm, complicating interpretations of polarization involving aligned grains. We find evidence against alignment by radiative flux in each source, particularly in the edge-on VLA 1623W and L1527. We detect astrometric offsets between the polarized emission and the total intensity in VLA 1623Aa, VLA 1623Ab, and VLA 1623B, as predicted if self-scattering in the optically thick limit operates. We conclude that self-scattering is likely responsible for disk-scale polarization at 872 μm in these systems.