Solving grain size inconsistency between ALMA polarization and VLA continuum in the Ophiuchus IRS 48 protoplanetary disk

TL;DR

This study reconciles the apparent contradiction between ALMA polarization data indicating small dust grains and VLA continuum data suggesting larger grains in the Ophiuchus IRS 48 disk by proposing optical thickness effects at 860 μm.

Contribution

The paper introduces a model where optical thickness at 860 μm explains the coexistence of small and large dust grain observations in the protoplanetary disk.

Findings

Polarized emission consistent with submillimeter self-scattering models.

Optically thick thermal emission at 860 μm suggests smaller observable grain size.

Larger dust grains may still be present near the midplane.

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 $\mu$m ALMA observations of polarized dust emission of 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 $\sim100$ $\mu$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 VLA observations, we suggest that the thermal emission at 860 $\mu$m wavelength is optically thick ($\tau_{\rm abs}\sim7.3$) at the dust trap with the maximum observable grain size of $\sim100$ $\mu$m rather than an optically thin case with $\sim$ cm dust grains. We note that we cannot rule out that larger dust grains are accumulated near the midplane if the 860 $\mu$m thermal emission is optically thick.