Polarimetric and radiative transfer modelling of HD 172555

TL;DR

This study combines polarimetric and radiative transfer modelling to analyze the dust grains in the debris disc around HD 172555, providing insights into grain size, composition, and disc structure.

Contribution

It introduces a comprehensive modelling approach that integrates polarimetric measurements with radiative transfer simulations to characterize dust in the HD 172555 disc.

Findings

Polarimetric measurements are consistent with recent SPHERE observations.

Dust grains are best modeled as spherical, around 1.2 μm or 3.9 μm depending on composition.

A single grain model can reproduce both the disc's emission and polarization data.

Abstract

The debris disc around HD 172555 was recently imaged in near-infrared polarised scattered light by the Very Large Telescope's Spectro-Polarimetric High-contrast Exoplanet REsearch instrument. Here we present optical aperture polarisation measurements of HD 172555 by the HIgh Precision Polarimetric Instrument (HIPPI), and its successor HIPPI-2 on the Anglo-Australian Telescope. We seek to refine constraints on the disc's constituent dust grains by combining our polarimetric measurements with available infrared and millimetre photometry to model the scattered light and continuum emission from the disc. We model the disc using the 3D radiative transfer code Hyperion, assuming the orientation and extent of the disc as obtained from the SPHERE observation. After correction for the interstellar medium contribution, our multi-wavelength HIPPI/-2 observations (both magnitude and orientation) are consistent with the recent SPHERE polarisation measurement with a fractional polarisation $p = 62.4 \pm 5.2$~ppm at 722.3 nm, and a position angle $\theta = 67 \pm 3^{\circ}$. The multi-wavelength polarisation can be adequately replicated by compact, spherical dust grains (i.e. from Mie theory) that are around 1.2 $\mu$m in size, assuming astronomical silicate composition, or 3.9 $\mu$m assuming a composition derived from radiative transfer modelling of the disc. We were thus able to reproduce both the spatially resolved disc emission and polarisation with a single grain composition model and size distribution.