Solving Grain Size Inconsistency between ALMA Polarization and VLA Continuum in the Ophiuchus IRS 48 Protoplanetary Disk
Satoshi Ohashi, Akimasa Kataoka, Nienke van der Marel, Charles L. H. Hull, William R. F. Dent, A. Pohl, Paola Pinilla, E. F. van Dishoeck, Thomas Henning
Abstract
Abstract 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 μ 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 μ 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 μ m wavelength is optically thick ( τ abs ∼ 7.3) at the dust trap with a maximum observable grain size of ∼100 μ 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 μ m thermal emission is optically thick.