High-resolution ALMA observations of V4046 Sgr: a circumbinary disc with a thin ring

The nearby V4046 Sgr spectroscopic binary hosts a gas-rich disc known for its wide cavity and dusty ring. We present high resolution (similar to 20 mas or 1.4 au) ALMA observations of the 1.3 mm 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 predictions. The ALMA data reveal a thin ring at a radius of 13.15 +/- 0.42 au (Ring13), with a radial width of 2.46 +/- 0.56 au. Ring13 is surrounded by a similar to 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 similar to 35 au the brightness of Ring24 is relatively flat and then breaks into a steep tail that reaches out to similar to 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 similar to 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

Automated summary

We present a physical model of the gas-rich disc of the nearby V4046 Sgr spectroscopic binary based on high resolution ALMA observations, SPHERE-IRDIS polarised images, and a well-sampled spectral energy distribution (SED). Our observations reveal the presence of thin rings (Ring13 and Ring24) with a wide gap in between, as well as a central emission interpreted as a tight circumbinary ring. The model also requires an inner ring (Ring5) composed mainly of small dust grains. The findings suggest a forward-scattering phase function and provide insights into the structure and composition of the disc.