SPHERE/ZIMPOL insights into discs around evolved stars: arcs, asymmetries and dust properties

TL;DR

The paper presents a multi-wavelength polarimetric study of five post-AGB circumbinary discs with SPHERE/ZIMPOL, revealing high optical/near-IR disc brightness and grey-to-blue polarimetric colours indicative of porous, aggregate dust on disc surfaces. By combining new optical polarimetry with archival near-IR results and radiative-transfer-informed interpretations, the authors constrain disc geometries, measure fractional polarisation up to DoLP ≈ 0.7 in AR Pup, and identify substructures such as arcs and asymmetries across targets. The results support a picture in which post-AGB discs host large, porous aggregates and share morphologies and polarimetric behaviour with protoplanetary discs around YSOs, while differing in dust processing histories and evolutionary context; some targets may be in transition between AGB and post-AGB stages. The study emphasizes the value of high-resolution, multi-wavelength polarimetry combined with radiative-transfer modelling to decode dust properties and disc evolution in evolved binaries, guiding future observations and theoretical work.

Abstract

Second-generation circumbinary discs around evolved binary stars, such as post-Asymptotic Giant Branch (post-AGB) binaries, provide insights into poorly understood mechanisms of dust processing and disc evolution across diverse stellar environments. We present a multi-wavelength polarimetric survey of five evolved binary systems - AR Pup, HR 4049, HR 4226, U Mon, and V709 Car - using the Very Large Telescope SPHERE/ZIMPOL instrument. Post-AGB discs show significant polarimetric brightness at optical and near-IR wavelengths, often exceeding 1% of the system's total intensity. We also measured a maximum fractional polarisation of the scattered light for AR Pup of ~0.7 in the V-band and ~0.55 in the I-band. To investigate wavelength-dependent polarisation, we combine the SPHERE/ZIMPOL dataset with results from previous SPHERE/IRDIS studies. This analysis reveals that post-AGB discs exhibit a grey to blue polarimetric colour in the optical and near-IR. Along with high fractional polarisation of the scattered light and polarised intensity distribution, these findings are consistent with a surface dust composition dominated by porous aggregates, reinforcing independent observational evidence for such grains in post-AGB circumbinary discs. We also find evidence of diverse disc geometries within the post-AGB sample, including arcs, asymmetries and significant variations in disc size across optical and near-IR wavelengths for some systems (U Mon, V709 Car). On comparing post-AGB discs to circumstellar environments around AGB stars and YSOs, we found that post-AGB systems exhibit a higher degree of polarisation than single AGB stars and are comparable to the brightest protoplanetary discs around YSOs.

Figures (23)

• Figure 1: Characteristics of unresolved central polarisation for scientific targets and polarised intensity of reference stars as a function of wavelength. The left panel shows the degree of unresolved polarisation (solid lines) relative to the total intensity of each target and the polarised intensity relative to the total intensity for each reference star (dashed lines). The right panel displays the corresponding orientation of unresolved polarisation (AoLP) for the targets (solid lines) and reference stars (dashed lines). Each target binary system and its corresponding reference star are indicated by matching colours. See Section \ref{['sec:paper3_data_reduction']} for more details.
• Figure 2: Total polarised intensity of all targets in $V-$ and $I'-$bands. The first and third columns display polarimetric images after standard PDI reduction and correction of unresolved polarisation (except for AR Pup, where the correction could not be reliably applied), while the second and fourth columns show images after additional deconvolution with PSF. White contours outline regions of statistically significant polarised intensity (SNR=3). White circles in the lower-left corner of each image indicate the size of the resolution element. All images are presented on an inverse hyperbolic scale and oriented North up and East to the left. See Section \ref{['sec:paper3_data_reduction']} for more details.
• Figure 3: Disc orientation results based on the polarimetric images of all targets (see Section \ref{['sec:paper3_disc_orient']} for more details). The red ellipses illustrate the most plausible PA and inclination of discs, while the dashed orange line highlights a significant substructure for HR 4049 (see Section \ref{['sec:paper3_substr']}). The red cross in the centre of the images represents the centre of the fitted ellipse, while the white cross represents the approximated position of the binary based on the PSF
• Figure 4: Percentage of total polarised disc intensity per resolved structure for all targets in $V-$ (top row) and $I'-$ (bottom row) bands (see Section \ref{['sec:paper3_substr']}). All images are presented on an inverse hyperbolic scale and oriented North up and East to the left. The low intensity of the central 5x5 pixel region of each image is a reduction bias caused by over-subtracting of unresolved central polarisation (Section \ref{['sec:paper3_data_reduction']}).
• Figure 5: Radial profiles of normalized total intensity ($I_{\rm tot}/I^{\rm max}_{\rm tot}$) for AR Pup (orange circles) and reference star HD 75885 (blue squares), and normalized polarised intensity ($I_{\rm pol}/I^{\rm max}_{\rm pol}$) for AR Pup (green stars). See Section \ref{['sec:paper3_scatter']} for details.