Revisiting symbiotic binaries with interferometry: II. New PIONIER data

TL;DR

This study uses interferometry and Gaia data to measure the sizes of red giants in symbiotic binaries, revealing most are wind-fed while one nearly fills its Roche lobe, advancing understanding of mass transfer mechanisms.

Contribution

It provides new interferometric measurements of giant radii in symbiotic binaries and assesses their mass transfer modes, highlighting the importance of stellar wind in most cases.

Findings

Most giants are within their Roche lobes, indicating wind accretion.

The giant in ZZ CMi nearly fills its Roche lobe, suggesting possible Roche-lobe overflow.

Interferometric data supports the classification of these giants as evolved, likely on the asymptotic giant branch.

Abstract

Symbiotic stars, which generally comprise a red giant and an accreting white dwarf, are excellent laboratories to understand mass transfer in wide binaries, with application to a wide family of systems. One of the fundamental questions is how mass is transferred from the red giant to the white dwarf. We use interferometric measurements made with the VLTI/PIONIER instrument, combined with Gaia data, to measure the radius of the giant in seven symbiotic systems. We further place the giants in the H-R diagramme, which allows us to estimate their mass and to show that they are all very evolved and likely on the asymptotic giant branch. We compare our measured giant radii to their Roche-lobe radius and show that, except for ZZ CMi, all giants are well within their Roche lobe and that mass transfer likely takes place via stellar wind. Our interferometric data provide further evidence that the giant in ZZ CMi (nearly) fills its Roche lobe. Our conclusions are still hampered by the poor characterisation of some of the giants or their binary orbit, and we encourage the community to make an effort to provide these.