Understanding post-red giant branch binaries through stable mass transfer

¹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010 6500 GL, Nijmegen, The Netherlands
² Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

^⋆ Corresponding author: casper.moltzer@ru.nl

Received: 16 July 2025
Accepted: 3 September 2025

Abstract

Context. Post-red giant branch (post-RGB) and post-asymptotic giant branch (post-AGB) binaries consist of a primary star that has recently evolved off either the RGB or AGB after losing the majority of its envelope and a main-sequence companion. They are distinguished by having luminosities below and above the tip of the RGB, respectively. These systems are characterised by the presence of a stable, dusty circumbinary disc, identified by a near-IR excess. Observed Galactic post-AGB and post-RGB binaries have orbital periods and eccentricities that are at odds with binary population synthesis models.

Aims. In this work, we focus on post-RGB binaries. We investigate whether stable mass transfer can explain the orbital periods of such binaries by comparing stable mass transfer models with the known sample of 38 Galactic post-RGB binaries.

Methods. We systematically determined the luminosities of the Galactic post-RGB and post-AGB binary sample using spectral energy distribution fitting. We computed evolution models for low- and intermediate-mass binaries with RGB donors at two metallicities using the detailed stellar evolution code, MESA. We selected the stable mass transfer models that result in primaries with effective temperatures within the observed range of post-RGB binaries (4000 − 8500 K).

Results. From our model grids, we find that low-mass post-RGB binaries are expected to follow strict luminosity–orbital period relations. The Galactic post-RGB binaries appear consistent with these luminosity–orbital period relations if we assume that their orbits remained eccentric during mass transfer and that the donor star filled its Roche lobe at periastron. However, our models are unable to explain the eccentricities themselves. Furthermore, the post-mass-transfer ages of observed post-RGB binaries estimated using our models are significantly longer than the predicted dissipation timescales of their circumbinary discs.

Conclusions. The stable mass transfer formation channel appears to explain the orbital periods of Galactic post-RGB binaries. This formation scenario could be tested more extensively by obtaining the orbits of additional Galactic systems, as well as those of the numerous candidates in the Magellanic Clouds, through long-term radial velocity monitoring. Additionally, we expect that Gaia Data Release 4 will improve the luminosities of Galactic post-RGB binaries, which will allow for a more accurate comparison with post-RGB luminosity–orbital period relations.