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Harris, Robert J.,Cox, Erin G.,Looney, Leslie W.,Li, Zhi-Yun,Yang, Haifeng,Ferná,ndez-Ló,pez, Manuel,Kwon, Woojin,Sadavoy, Sarah,Segura-Cox, Dominique,Stephens, Ian,Tobin, John American Astronomical Society 2018 The Astrophysical journal Vol.861 No.2
<P>We present high-sensitivity (sigma(I) similar to 0.2-0.5 mJy, sigma(QU) similar to 0.05 mJy), high-resolution (similar to 0.'' 12-0.'' 2) observations of polarized 872 mu m dust emission from the young multiple system VLA 1623 in rho 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 similar to 1%-3% level. We spatially resolve VLA 1623A into two sources, VLA 1623Aa and VLA 1623Ab, separated by similar to 30 au and located within a cavity of radius similar to 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 less than or similar to 2 x 10(-4) level. The circumbinary disk surrounding VLA 1623Aab is azimuthally symmetrically polarized. Each compact source's emission is partially optically thick (tau greater than or similar to 1) at 872 mu m, 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 mu m in these systems.</P>
Dust Polarization toward Embedded Protostars in Ophiuchus with ALMA. II. IRAS 16293-2422
Sadavoy, Sarah I.,Myers, Philip C.,Stephens, Ian W.,Tobin, John,Kwon, Woojin,Segura-Cox, Dominique,Henning, Thomas,Commerç,on, Benoî,t,Looney, Leslie American Astronomical Society 2018 The Astrophysical journal Vol.869 No.2
ALMA Reveals Transition of Polarization Pattern with Wavelength in HL Tau’s Disk
Stephens, Ian W.,Yang, Haifeng,Li, Zhi-Yun,Looney, Leslie W.,Kataoka, Akimasa,Kwon, Woojin,Ferná,ndez-Ló,pez, Manuel,Hull, Charles L. H.,Hughes, Meredith,Segura-Cox, Dominique,Mundy, Lee,C American Astronomical Society 2017 The Astrophysical journal Vol.851 No.1
<P>The mechanism for producing polarized emission from protostellar disks at (sub) millimeter wavelengths is currently uncertain. Classically, polarization is expected from non-spherical grains aligned with the magnetic field. Recently, two alternatives have been suggested. One polarization mechanism is caused by self-scattering from dust grains of sizes comparable with the wavelength, while the other mechanism is due to grains aligned with their short axes along the direction of radiation anisotropy. The latter has recently been shown as a likely mechanism for causing the dust polarization detected in HL. Tau at 3.1 mm. In this paper, we present ALMA polarization observations of HL. Tau for two more wavelengths: 870 mu m and 1.3 mm. The morphology at 870 mu m matches the expectation for self-scattering, while that at 1.3 mm shows a mix between self-scattering and grains aligned with the radiation anisotropy. The observations cast doubt on the ability of (sub) millimeter continuum polarization to probe disk magnetic fields for at least HL Tau. By showing two distinct polarization morphologies at 870 mu m and 3.1 mm and a transition between the two at 1.3 mm, this paper provides definitive evidence that the dominant (sub) millimeter polarization mechanism transitions with wavelength. In addition, if the polarization at 870 mu m is due to scattering, the lack of polarization asymmetry along the minor axis of the inclined disk implies that the large grains responsible for the scattering have already settled into a geometrically thin layer, and the presence of asymmetry along the major axis indicates that the HL Tau disk is not completely axisymmetric.</P>
Dust Polarization toward Embedded Protostars in Ophiuchus with ALMA. I. VLA 1623
Sadavoy, Sarah I.,Myers, Philip C.,Stephens, Ian W.,Tobin, John,Commerç,on, Benoî,t,Henning, Thomas,Looney, Leslie,Kwon, Woojin,Segura-Cox, Dominique,Harris, Robert American Astronomical Society 2018 The Astrophysical journal Vol.859 No.2
<P>We present high-resolution (similar to 30 au) ALMA Band 6 dust polarization observations of VLA 1623. The VLA 1623 data resolve compact similar to 40 au inner disks around the two protobinary sources, VLA 1623-A and VLA 1623-B, and also an extended similar to 180 au ring of dust around VLA 1623-A. This dust ring was previously identified as a large disk in lower-resolution observations. We detect highly structured dust polarization toward the inner disks and the extended ring with typical polarization fractions similar to 1.7% and similar to 2.4%, respectively. The two components also show distinct polarization morphologies. The inner disks have uniform polarization angles aligned with their minor axes. This morphology is consistent with expectations from dust scattering. By contrast, the extended dust ring has an azimuthal polarization morphology not previously seen in lower-resolution observations. We find that our observations are well-fit by a static, oblate spheroid model with a flux-frozen, poloidal magnetic field. We propose that the polarization traces magnetic grain alignment likely from flux freezing on large scales and magnetic diffusion on small scales. Alternatively, the azimuthal polarization may be attributed to grain alignment by the anisotropic radiation field. If the grains are radiatively aligned, then our observations indicate that large (similar to 100 mu m) dust grains grow quickly at large angular extents. Finally, we identify significant proper motion of VLA 1623 using our observations and those in the literature. This result indicates that the proper motion of nearby systems must be corrected for when combining ALMA data from different epochs.</P>