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

Polarimetric and radiative transfer modelling of HD 172555