The polarisation properties of the HD 181327 debris ring. Evidence for sub-micron particles from scattered light observations
arxiv(2023)
摘要
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. We aim to measure the
scattering properties of the dust from the debris ring around HD 181327 at
near-infrared wavelengths. 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 F110W 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. 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 micron in the birth ring. The maximum polarisation is 23.6% +/- 2.6% and
occurs between a scattering angle of 70 deg and 82 deg. 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.
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