# Intrinsic polarisation of elongated porous dust grains

## TL;DR

This paper investigates how the intrinsic polarisation of elongated porous dust grains in protoplanetary disks varies with wavelength and porosity, providing a new method to estimate grain properties from polarisation observations.

## Contribution

It introduces a novel analysis of the impact of porosity on dust grain polarisation and proposes a method to constrain grain porosity and size using multi-wavelength polarisation data.

## Key findings

- Intrinsic polarisation decreases with increasing grain porosity.
- Polarisation orientation can flip by 90 degrees depending on wavelength-to-grain size ratio.
- Moderate porosity grains can explain observed polarisation in HD 142527.

## Abstract

ALMA observations revealed recently polarised radiation of several protoplanetary disks in the (sub-)millimetre wavelength range. Besides self-scattering of large particles, thermal emission by elongated grains is a potential source for the detected polarisation signal. We calculate the wavelength dependent absorption and intrinsic polarisation of spheroidally shaped, micrometre and sub-millimetre sized dust grains using the discrete dipole approximation. In particular, we analyse the impact of dust grain porosity which appears to be present in disks when small grains coagulate to form larger aggregates. For the first time our results show that (a) the intrinsic polarisation decreases for increasing grain porosity and (b) the polarisation orientation flips by 90 degree for certain ratios of wavelength to grain size. We present a new method to constrain grain porosity and the grain size in protoplanetary disks using multi-wavelength polarisation observations in the far-infrared to millimetre wavelengths. Finally, we find that moderate grain porosities ($\mathcal{P}\lesssim0.7$) potentially explain the observed polarisation fraction in the system HD 142527 while highly porous grains ($\mathcal{P}>0.7$) fail unless the grain's axis ratio is extraordinarily large.