The polarisation properties of the HD 181327 debris ring

Evidence for sub-micron particles from scattered light observations^★

¹ Université Grenoble-Alpes, CNRS, IPAG, 38000 Grenoble, France
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² Aix-Marseille Univ., CNRS, CNES, LAM, Marseille, France
³ Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁵ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁶ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris-Diderot, Sorbonne Paris-Cité, 5 place Jules Janssen, 92195 Meudon, France
⁷ LATMOS, Sorbonne Université, CNRS, CNES, Paris, France
⁸ IRAP, Université de Toulouse, CNES, CNRS, UPS, Toulouse, France
⁹ LPC2E, Université d’Orléans, CNRS, Orléans, France
¹⁰ LATMOS, CNRS, 11 bld d’Alembert, 78280 Guyancourt, France
¹¹ European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile
¹² Planetary Atmospheres Group, Institute for Basic Science (IBS), 55 Expo-ro, Yuseong-gu, Daejeon 34126, South Korea
¹³ Department of Physics and Astronomy, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK
¹⁴ Anton Pannekoek Institute for Astronomy, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹⁵ CRAL, UMR 5574, CNRS, Université de Lyon, Ecole Normale Supérieure de Lyon, 46 Allée d’Italie, 69364 Lyon Cedex 07, France
¹⁶ Millennium Nucleus on Young Exoplanets and their Moons (YEMS), Chile
¹⁷ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
¹⁸ Escuela de Ingeniería Industrial, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
¹⁹ DOTA, ONERA, 13661, Salon Cedex Air, France

Received: 11 September 2023
Accepted: 22 November 2023

Abstract

Context. Polarisation is a powerful remote-sensing tool to study the nature of particles scattering the starlight. It is widely used to characterise interplanetary dust particles in the Solar System and increasingly employed to investigate extrasolar dust in debris discs’ systems.

Aims. We aim to measure the scattering properties of the dust from the debris ring around HD 181327 at near-infrared wavelengths.

Methods. We obtained high-contrast polarimetric images of HD 181327 in the H band with the SPHERE/IRDIS instrument on the Very Large Telescope (ESO). We complemented them with archival data from HST/NICMOS in the F 110W filter reprocessed in the context of the Archival Legacy Investigations of Circumstellar Environments (ALICE) project. We developed a combined forward-modelling framework to simultaneously retrieve the scattering phase function in polarisation and intensity.

Results. We detected the debris disc around HD 181327 in polarised light and total intensity. We measured the scattering phase function and the degree of linear polarisation of the dust at 1.6 µm in the birth ring. The maximum polarisation is 23.6% ± 2.6% and occurs between a scattering angle of 70° and 82°.

Conclusions. We show that compact spherical particles made of a highly refractive and relatively absorbing material in a differential power-law size distribution of exponent −3.5 can simultaneously reproduce the polarimetric and total intensity scattering properties of the dust. This type of material cannot be obtained with a mixture of silicates, amorphous carbon, water ice, and porosity, and requires a more refracting component such as iron-bearing minerals. We reveal a striking analogy between the near-infrared polarisation of comets and that of HD 181327. The methodology developed here combining VLT/SPHERE and HST/NICMOS may be applicable in the future to combine the polarimetric capabilities of SPHERE with the sensitivity of JWST.