The effects of disk and dust structure on observed polarimetric images of protoplanetary disks

TL;DR

This study investigates how dust particle properties and instrumental effects influence the interpretation of polarimetric images of protoplanetary disks, emphasizing the need for realistic modeling and proper calibration.

Contribution

It demonstrates the significant impact of dust particle shape, size, and disk structure on polarimetric observations and highlights the importance of realistic dust models and adaptive optics for accurate analysis.

Findings

Particle shape and size strongly affect brightness and detectability.

Inner disk regions contribute to polarization in the central resolution element.

Homogeneous sphere models differ significantly from realistic dust models.

Abstract

Imaging polarimetry is a powerful tool for imaging faint circumstellar material. For a correct analysis of observations we need to fully understand the effects of dust particle parameters, as well as the effects of the telescope, atmospheric seeing, and assumptions about the data reduction and processing of the observed signal. Here we study the major effects of dust particle structure, size-dependent grain settling, and instrumental properties. We performed radiative transfer modeling using different dust particle models and disk structures. To study the influence of seeing and telescope diffraction we ran the models through an instrument simulator for the ExPo dual-beam imaging polarimeter mounted at the 4.2m William Herschel Telescope (WHT). Particle shape and size have a strong influence on the brightness and detectability of the disks. In the simulated observations, the central resolution element also contains contributions from the inner regions of the protoplanetary disk besides the unpolarized central star. This causes the central resolution element to be polarized, making simple corrections for instrumental polarization difficult. This effect strongly depends on the spatial resolution, so adaptive optics systems are needed for proper polarization calibration. We find that the commonly employed homogeneous sphere model gives results that differ significantly from more realistic models. For a proper analysis of the wealth of data available now or in the near future, one must properly take the effects of particle types and disk structure into account. The observed signal depends strongly on the properties of these more realistic models, thus providing a potentially powerful diagnostic. We conclude that it is important to correctly understand telescope depolarization and calibration effects for a correct interpretation of the degree of polarization.