Resolving the polarized dust emission of the disk around the massive star powering the HH~80-81 radio jet

TL;DR

This study uses high-resolution ALMA observations to analyze the polarized dust emission of a massive protostar's disk, revealing distinct polarization patterns and dust properties indicative of different optical depths and dust grain sizes.

Contribution

First high-resolution polarimetric imaging of a massive protostar's disk, showing polarization differentiation between optically thick and thin regions and insights into dust grain sizes.

Findings

Inner disk shows no dust settling, with grain sizes between 50-500 micrometers.

Outer disk exhibits azimuthal polarization pattern consistent with self-scattering.

Transition at ~170 au marks change in polarization and optical depth.

Abstract

Here we present deep (16 mumJy), very high (40 mas) angular resolution 1.14 mm, polarimetric, Atacama Large Millimeter/submillimeter Array (ALMA) observations towards the massive protostar driving the HH 80-81 radio jet. The observations clearly resolve the disk oriented perpendicular to the radio jet, with a radius of ~0.171 arcsec (~291 au at 1.7 kpc distance). The continuum brightness temperature, the intensity profile, and the polarization properties clearly indicate that the disk is optically thick for a radius of R<170 au. The linear polarization of the dust emission is detected almost all along the disk and its properties suggest that dust polarization is produced mainly by self-scattering. However, the polarization pattern presents a clear differentiation between the inner (optically thick) part of the disk and the outer (optically thin) region of the disk, with a sharp transition that occurs at a radius of 0.1 arcsec (~170 au). The polarization characteristics of the inner disk suggest that dust settling has not occurred yet with a maximum dust grain size between 50 and 500 mum. The outer part of the disk has a clear azimuthal pattern but with a significantly higher polarization fraction compared to the inner disk. This pattern is broadly consistent with self-scattering of a radiation field that is beamed radially outward, as expected in the optically thin outer region, although contribution from non-spherical grains aligned with respect to the radiative flux cannot be excluded.