High-resolution ALMA observations of V4046Sgr: a circumbinary disc with a thin ring

TL;DR

This study uses high-resolution ALMA and other observations to reveal a detailed structure of the V4046 Sgr circumbinary disc, including a thin ring, gaps, and multiple dust components, providing insights into disc morphology and dust properties.

Contribution

First high-resolution ALMA imaging combined with radiative transfer modeling to characterize the complex structure of the V4046 Sgr circumbinary disc.

Findings

Discovery of a thin ring at 13.15 au with a narrow width.

Identification of a gap and an outer bright ring with sharp edges.

Detection of a central dust ring close to the binary at 1.1 au.

Abstract

The nearby V4046 Sgr spectroscopic binary hosts a gas-rich disc known for its wide cavity and dusty ring. We present high resolution ($\sim$20 mas or 1.4 au) ALMA observations of the 1.3mm continuum of V4046 Sgr which, combined with SPHERE--IRDIS polarised images and a well-sampled spectral energy distribution (SED), allow us to propose a physical model using radiative transfer (RT) predictions. The ALMA data reveal a thin ring at a radius of 13.15$\pm$0.42 au (Ring13), with a radial width of 2.46$\pm$0.56 au. Ring13 is surrounded by a $\sim$10 au-wide gap, and it is flanked by a mm-bright outer ring (Ring24) with a sharp inner edge at 24 au. Between 25 and $\sim$35 au the brightness of Ring24 is relatively flat and then breaks into a steep tail that reaches out to $\sim$60 au. In addition, central emission is detected close to the star which we interpret as a tight circumbinary ring made of dust grains with a lower size limit of 0.8 mm at 1.1 au. In order to reproduce the SED, the model also requires an inner ring at $\sim$5 au (Ring5) composed mainly of small dust grains, hiding under the IRDIS coronagraph, and surrounding the inner circumbinary disc. The surprisingly thin Ring13 is nonetheless roughly 10 times wider than its expected vertical extent. The strong near-far disc asymmetry at 1.65 $\mu$m points at a very forward-scattering phase function and requires grain radii of no less than 0.4 $\mu$m.