Revisiting symbiotic binaries with interferometry. I. The PIONIER archival collection

TL;DR

This study uses interferometry and Gaia data to analyze six symbiotic stars, finding their giants are within Roche lobes, which sheds light on mass transfer mechanisms in these binary systems.

Contribution

It provides the first detailed interferometric analysis of these symbiotic stars, clarifying their stellar radii and Roche lobe filling factors with new observational data.

Findings

Giants are within their Roche lobes in all studied systems.

Most giants are luminous and likely on the asymptotic giant branch.

V399 Pav remains uncertain regarding its Roche lobe status.

Abstract

Symbiotic stars serve as exceptional laboratories for investigating mass transfer processes in binary systems. However, the dominant mechanism of mass transfer from the red giant donor to the compact accretor - typically a white dwarf or, in rare cases, a neutron star - remains unclear. It is uncertain whether it is driven primarily by the stellar wind, Roche-lobe overflow, or a combination of the two. While radii inferred from rotational velocities or spectral types suggest smaller Roche-lobe filling factors, the presence of ellipsoidal variability, presumably caused by tidally deformed giants in many symbiotic systems, indicates the opposite. Interferometric observations of symbiotic giants, combined with distance measurements provided by the Gaia mission, offer a promising avenue to resolve this discrepancy. In this first paper of the series, we (re)analyze VLTI/PIONIER observations of six symbiotic stars: AG Peg, FG Ser, ER Del, V1261 Ori, RW Hya, and V399 Pav. With the exception of the uncertain case of V399 Pav, we find that the giants in these systems remain well within their canonical Roche lobes, even in V1261 Ori and RW Hya, where ellipsoidal variability is observed. All six stars appear to be rather luminous and likely located on the asymptotic giant branch, although the possibility of some of them being at the tip of the first red giant branch cannot be ruled out.