Jonathan Marshall (Academia Sinica, Institute of Astronomy and Astrophysics, 11F Astronomy-Mathematics Building, NTU/AS campus, No. 1, Section 4, Roosevelt Rd., Taipei 10617, Taiwan); Daniel V. Cotton (University of Southern Queensland, Centre for Astrophysics, Toowoomba, QLD 4350, Australia); Peter Scicluna (European Southern Observatory, Alonso de Cordova 3107, Santiago RM, Chile); Jeremy Bailey (School of Physics, University of New South Wales, Sydney, NSW 2052, Australia); Lucyna Kedziora-Chudczer (University of Southern Queensland, Centre for Astrophysics, Toowoomba, QLD 4350, Australia); Kimberly Bott (Department of Earth and Planetary Science, University of California, Riverside, CA, 92521, USA)
The debris disc around HD 172555 was recently imaged in near-infrared polarised scattered light by the Very Large Telescope's instrument Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE). 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 ± 5.2 ppm at 722.3 nm, and a position angle θ= 67 ± 3°. The multi-wavelength polarisation can be adequately replicated by compact, spherical dust grains (i.e. from Mie theory) that are around 1.2 μm in size, assuming astronomical silicate composition, or 3.89 μm 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.