Turbulent vortex with moderate dust settling probed by scattering-induced polarization in the IRS 48 system

Abstract

We investigate the crescent-shaped dust trap in the transition disk, Oph IRS 48, using well-resolved (sub)millimeter polarimetric observations at ALMA Band 7 (870 $\mu$m). The dust polarization map reveals patterns consistent with dust scattering-induced polarization. There is a relative displacement between the polarized flux and the total flux, which holds the key to understanding the dust scale heights in this system. We model the polarization observations, focusing on the effects of dust scale heights. We find that the interplay between the inclination-induced polarization and the polarization arising from radiation anisotropy in the crescent determines the observed polarization; the anisotropy is controlled by the dust optical depth along the midplane, which is, in turn, determined by the dust scale height in the vertical direction. We find that the dust grains can neither be completely settled nor well mixed with the gas. The completely settled case produces little radial displacement between the total and polarized flux, while the well-mixed case produces an azimuthal pattern in the outer (radial) edge of the crescent that is not observed. Our best model has a gas-to-dust scale height ratio of 2, and can reproduce both the radial displacement and the azimuthal displacement between the total and polarized flux. We infer an effective turbulence $\alpha$ parameter of approximately $0.0001-0.005$. The scattering-induced polarization provides insight into a turbulent vortex with a moderate level of dust settling in the IRS 48 system, which is hard to achieve otherwise.