Understanding post-red giant branch binaries through stable mass transfer

Abstract

Post-RGB and post-AGB binaries consist of a primary star that has recently evolved off either the RGB or AGB after losing most of its envelope, and a main-sequence companion. They are distinguished by luminosities below and above the RGB tip, respectively. These systems host a stable, dusty circumbinary disc, characterised by a near-infrared excess. Observed Galactic post-AGB and post-RGB binaries have orbital periods and eccentricities inconsistent with binary population synthesis models. Here, we focus on post-RGB binaries, testing whether stable mass transfer can explain their orbital periods by comparing models with the known sample of 38 Galactic post-RGB binaries. We systematically determined luminosities of Galactic post-RGB and post-AGB binaries through SED fitting. We computed evolution models for low- and intermediate-mass binaries with RGB donors at two metallicities using MESA. We selected stable mass transfer models producing primaries with effective temperatures within the observed range. From these models, we find that low-mass post-RGB binaries should follow strict luminosity-orbital period relations. The Galactic post-RGB binaries seem consistent with these relations if their orbits remained eccentric during mass transfer and if the donor filled its Roche lobe at periastron. However, our models are unable to explain the eccentricities themselves. Moreover, post-mass-transfer ages from our models are much longer than predicted dissipation timescales of circumbinary discs. Stable mass transfer seems to explain the orbital periods of Galactic post-RGB binaries. This formation channel can be tested further by obtaining orbits of additional Galactic systems and Magellanic Cloud candidates via long-term radial velocity monitoring. Gaia DR 4 will improve luminosities of Galactic post-RGB binaries, enabling more accurate comparison with luminosity-orbital period relations.

Figures (9)

• Figure 1: Eccentricity-orbital period diagram of the Galactic post-AGB and post-RGB binary sample.
• Figure 2: HRD of the Galactic post-AGB and post-RGB binary sample. Different markers denote whether the orbital periods of the objects are known. The $T_\mathrm{eff}$ error bars of $\pm250$ K were omitted for the sake of clarity. Evolutionary tracks of our single-star models with $Z=0.02$, $M_\mathrm{i}=1.5$$M_{\odot}$ and $Z=0.00142$, $M_\mathrm{i}=1.0$$M_{\odot}$ are shown in green and black, respectively, up to the final AGB evolution. In addition, we show a few illustrative post-RGB evolution tracks with initial orbital periods ranging from 3.95 days to 258 days (from bottom to top). These models have donors identical to their starting point on the corresponding single-star evolutionary track and have mass ratios equal to unity prior to mass transfer. Orbital periods of the sample binaries as well as the orbital period evolution of the models are shown by the colour bar. The dashed grey line corresponds to the RGB-tip luminosity of 2500 $L_{\odot}$.
• Figure 3: HRD of the post-AGB and post-RGB binary candidate samples in the LMC and SMC. The $T_{\mathrm{eff}}$ error bars of ±250 K were omitted for the sake of clarity. Also shown are evolutionary tracks of single star models, taken from the MIST database Choi2016, with $M_i/M_{\odot}=1.0$ and the [Fe/H] of $-0.30$ and $-0.65$ corresponding to the mean metallicities of the LMC and SMC, respectively Westerlund1997Larsen2000. The dashed grey line corresponds to the RGB-tip luminosity of 2500 $L_{\odot}$.
• Figure 4: Orbital period-luminosity diagram of the donor stars in our models following stable mass transfer with effective temperatures in the range of $4000-8500$ K, colour-coded by initial primary mass. Panel (a) corresponds to the solar metallicity grid, and panel (b) to the metal-poor grid. For those models that do not exhibit constant luminosity after mass transfer, the lines depict the variation in luminosity as the model traverses the effective temperature range, with the markers denoting the time-weighted average of the corresponding evolution. Different markers denote the type of post-mass-transfer donor: post-RGB, central He-burning, post-AGB, or pre-subdwarf (pre-sd).
• Figure 5: Orbital period-luminosity plot of our post-RGB models with initial primary masses below $2.0$$M_{\odot}$, indicated by markers and colour-coded by metallicity. Corresponding fits for the solar and metal-poor grids, given in Eq. \ref{['equation:LPorb']}, are denoted by solid lines.