Exploring Polarized Millimeter Emission from Protoplanetary Disks with Irregular Dust Grains

TL;DR

This study examines how irregular dust grain shapes influence polarized millimeter emission in protoplanetary disks, revealing that grain geometry affects scattering opacity and polarization features but alone cannot fully explain observations.

Contribution

It compares spherical and irregular grain models to isolate geometric effects, highlighting increased scattering opacity and polarization suppression due to irregular shapes.

Findings

Irregular grains increase scattering opacity by up to 2.5 times compared to spherical grains.

Grain shape affects polarization morphology and suppresses polarization reversal.

Modifying grain geometry alone cannot fully reproduce observed polarization fractions.

Abstract

Polarization at millimeter wavelengths provides a powerful diagnostic of dust grain properties in protoplanetary disks. Standard models based on solid spherical grains often struggle to reproduce the observed polarization fractions and morphologies in systems where self-scattering is expected to dominate. We investigate the impact of grain morphology on polarized millimeter emission by comparing models that adopt solid spherical grains with models that employ solid irregular hexahedral particles drawn from the TAMUdust2020 database. Both grain populations share identical size distributions, enabling us to isolate the effects of geometry while preserving the same internal structure and material density. We explore three optical-depth regimes-optically thick, optically thin, and an intermediate hybrid case-to assess how grain morphology modifies the polarization structure under different conditions. For size distributions with $a_{\mathrm{max}} \sim \lambda / 2\pi$, where scattering-induced polarization is expected to peak, we find that the polarization morphology and fraction are nearly indistinguishable between spherical and irregular grains. The primary quantitative difference is an enhancement of the scattering opacity by up to a factor of $\sim 2.5$ for irregular particles, implying that disk dust masses inferred under the assumption of spherical grains may be systematically overestimated. Irregular grains also suppress the polarization reversal predicted by Mie theory at large size parameters ($x>1$). Nevertheless, modifying grain geometry alone is insufficient to reproduce the observed polarization fractions within a pure self-scattering framework. These results suggest that additional physical effects, such as dust porosity, warrant dedicated investigation.