Solving Grain Size Inconsistency between ALMA Polarization and VLA Continuum in the Ophiuchus IRS 48 Protoplanetary Disk

• Published in: The Astrophysical journal 2020-09, Vol.900 (1), p.81
• Main Authors: Ohashi, Satoshi, Kataoka, Akimasa, van der Marel, Nienke, Hull, Charles L. H., Dent, William R. F., Pohl, Adriana, Pinilla, Paola, van Dishoeck, Ewine F., Henning, Thomas
• Format: Article
• Language: English
• Subjects: Astrophysics; Dust; Dust emission; Dust particles; Grain size; Mathematical analysis; Particle size; Planet formation; Polarimetry; Polarization; Protoplanetary disks; Skewed distributions; Submillimeter waves; Thermal emission; Vortices
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/abaab4

The protoplanetary disk around Ophiuchus IRS 48 shows an azimuthally asymmetric dust distribution in (sub)millimeter observations, which is interpreted as a vortex, where millimeter/centimeter-sized particles are trapped at the location of the continuum peak. In this paper, we present 860 m ALMA observations of polarized dust emission from this disk. The polarized emission was detected toward a part of the disk. The polarization vectors are parallel to the disk minor axis, and the polarization fraction was derived to be 1%-2%. These characteristics are consistent with models of self-scattering of submillimeter-wave emission, which indicate a maximum grain size of ∼100 m. However, this is inconsistent with the previous interpretation of millimeter/centimeter dust particles being trapped by a vortex. To explain both ALMA polarization and previous ALMA and Very Large Array observations, we suggest that the thermal emission at 860 m wavelength is optically thick (τabs ∼ 7.3) at the dust trap with a maximum observable grain size of ∼100 m rather than an optically thin case with centimeter-sized dust grains. We note that we cannot rule out that larger dust grains are accumulated near the midplane if the 860 m thermal emission is optically thick.