Self-scattering on large, porous grains in protoplanetary disks with dust settling

TL;DR

This study explores how dust grain porosity affects scattering-induced polarisation in protoplanetary disks, revealing that porous grains can increase polarisation but still fall short of observed levels unless certain model assumptions are relaxed.

Contribution

It demonstrates that dust grain porosity influences polarisation in protoplanetary disks and highlights the importance of dust settling and grain size assumptions for matching observations.

Findings

Moderate porosity (~10%) increases polarisation compared to compact grains.

Highly porous grains (<1% filling factor) reduce polarisation due to decreased optical depth.

Inclined disks with higher masses can achieve polarisation levels around 1%.

Abstract

Observations of protoplanetary disks (PPDs) at sub-mm wavelengths suggest that polarisation is caused by scattering of thermal radiation. Most of the dust models that are used to explain these observations have major drawbacks: They either use much smaller grains than expected from dust evolution models, or result in lower polarisations than observed. We investigate the effect of dust grain porosity on the polarisation due to scattering at sub-mm wavelengths arising from grains of up to mm sizes, as they are expected to be present in the midplane of PPDs. Using the effective medium theory, we calculated the optical properties of porous dust and used them to predict the behaviour of the scattering polarisation. Subsequently, radiative transfer simulations for PPDs with porous dust were performed to analyse the additional effect of the optical depth structure, and thus the effect of multiple scattering events and inhomogeneous temperature distributions on the polarisation. We find that porous dust with moderate filling factors of about 10\% increase the polarisation compared to compact grains. For higher grain porosities, that is, grains with a filling factor of 1\% or lower, the extinction opacity decreases, as does the optical depth of a disk with constant mass. Consequently, the unpolarised direct radiation dominates the scattered flux, and the polarisation drops rapidly. Even though the simulated polarisation is higher than in the case of compact grains, it is still below the typical observed values for face-on disks. However, the polarisation can be increased when crucial model assumptions derived from disk and dust evolution theories, for instance, dust settling and mm-sized dust grains, are dropped. In the case of inclined disks, however, our reference model is able to achieve polarisation degrees of about 1\%, and using higher disk masses, even higher than this.