ALMA Observations of Polarized Emission toward the CW Tau and DG Tau Protoplanetary Disks: Constraints on Dust Grain Growth and Settling

We present polarimetric data of CW Tau and DG Tau, two well-known Class II disk/jet systems, obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) at 870 m and 0 2 average resolution. In CW Tau, the total and polarized emission are both smooth and symmetric, with polarization angles...

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Bibliographic Details
Published in:Astrophysical journal. Letters 2018-10, Vol.865 (2), p.L12
Main Authors: Bacciotti, Francesca, Miquel Girart, Josep, Padovani, Marco, Podio, Linda, Paladino, Rosita, Testi, Leonardo, Bianchi, Eleonora, Galli, Daniele, Codella, Claudio, Coffey, Deirdre, Favre, Cecile, Fedele, Davide
Format: Article
Language:English
Subjects:
Angles (geometry)
Disks
Dust
Elongated structure
Emission analysis
Grain growth
ISM: jets and outflows
Opacity
Optical analysis
Optical thickness
Planet formation
Polarimetry
Polarization
Protoplanetary disks
Protoplanets
Radio telescopes
Sciences of the Universe
Settling
Thermal emission
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Summary:We present polarimetric data of CW Tau and DG Tau, two well-known Class II disk/jet systems, obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) at 870 m and 0 2 average resolution. In CW Tau, the total and polarized emission are both smooth and symmetric, with polarization angles almost parallel to the minor axis of the projected disk. In contrast, DG Tau displays a structured polarized emission, with an elongated brighter region in the disk's near side and a belt-like feature beyond about 0 3 from the source. At the same time, the total intensity is spatially smooth, with no features. The polarization pattern, almost parallel to the minor axis in the inner region, becomes azimuthal in the outer belt, possibly because of a drop in optical depth. The polarization fraction has average values of 1.2% in CW Tau and 0.4% in DG Tau. Our results are consistent with polarization from self-scattering of the dust thermal emission. In this hypothesis, the maximum size of the grains contributing to polarization is in the range 100-150 m for CW Tau and 50-70 m for DG Tau. The polarization maps combined with dust opacity estimates indicate that these grains are distributed in a geometrically thin layer in CW Tau, representing a settling in the disk midplane. Meanwhile, such settling is not yet apparent for DG Tau. These results advocate polarization studies as a fundamental complement to total emission observations, in investigations of the structure and the evolution of protoplanetary disks.
ISSN:2041-8205
2041-8213
DOI:10.3847/2041-8213/aadf87