Self-scattering in protoplanetary disks with dust settling

TL;DR

This study investigates how dust settling and grain size distribution in protoplanetary disks influence the polarization of scattered sub-millimeter radiation, highlighting the complexity of interpreting polarization data.

Contribution

It introduces detailed radiative transfer simulations considering dust segregation and grain size diversity, advancing understanding of polarization mechanisms in disks.

Findings

Polarization degree varies with disk parameters affecting radiation anisotropy and optical depth.

High dust albedo can help identify transitions from optically thick to thin regions.

Model polarization levels are generally lower than observed, suggesting revisions in dust and disk models.

Abstract

Scattering of re-emitted flux is considered to be at least partially responsible for the observed polarisation in the (sub-)millimetre wavelength range of several protoplanetary disks. Although the degree of polarisation produced by scattering is highly dependent on the dust model, early studies investigating this mechanism relied on the assumption of single grain sizes and simple density distribution of the dust. However, in the dense inner regions where this mechanism is usually most efficient, the existence of dust grains with sizes ranging from nanometres to millimetres has been confirmed. Additionally, the presence of gas forces larger grains to migrate vertically towards the disk midplane, introducing a dust segregation in the vertical direction. Using polarisation radiative transfer simulations, we analyse the dependence of the resulting scattered light polarisation at 350 $\mu$m, 850 $\mu$m, 1.3 mm, and 2 mm on various parameters describing protoplanetary disks, including the effect of dust grain settling. We find that the different disk parameters change the degree of polarisation mostly by affecting the anisotropy of the radiation field, the optical depth, or both. It is therefore very challenging to deduce certain disk parameter values directly from polarisation measurements alone. However, assuming a high dust albedo, it is possible to trace the transition from optically thick to optically thin disk regions. The degree of polarisation in most of the considered disk configurations is lower than what is found observationally, implying the necessity to revisit models that describe the dust properties and disk structure.