Good things come to those who wait: Watching donor stars evolve towards a mass-transfer instability

TL;DR

The paper addresses how donor stars in binaries with Hertzsprung-gap evolution approach a mass-transfer instability, a phase relevant to luminous red novae (LRNe). It employs detailed MESA binary-evolution models across donor masses $2.5-10\,M_\odot$ and mass ratios $q$ spanning late-HG to stable regimes to predict a characteristic pre-DDI track: a long slow dimming followed by a brief rapid brightening powered by hydrogen recombination and accompanied by an increase in $T_{\rm eff}$. The results identify observable pre-DDI features, quantify timescales (e.g., $t_{DF}\sim 10-500$ years for the rapid brightening phase, $\Delta t_{\rm dim}\sim 10^4$–$10^5$ years), and show Gaia could detect many pre-DDI systems across the Galaxy; they also apply the framework to the V838 Mon and M31-2015 LRNe progenitors, suggesting higher donor masses than previous single-star inferences and proposing new progenitor scenarios. Overall, the work provides a comprehensive theory-to-observation linkage for mass-transfer transients, enabling improved classification and prediction of DDI-driven events in binaries.

Abstract

Unstable mass transfer in binary systems can lead to transients such as luminous red novae (LRNe). Observations of such transients are valuable for understanding and testing models of mass transfer. For donor stars in the Hertzsprung gap, there can be a long phase of mass-transfer evolution before instability sets in. Only few case studies of such delayed dynamical instability (DDI) mass transfer exist. None consider the full pre-instability evolution and the effects thereof on the observable properties of a binary. We systematically analyse detailed models of stable and unstable mass transfer for Hertzsprung-gap donors. We focus on identifying observable evolutionary features characteristic of ultimately unstable mass transfer and not found in stable mass-transfer binaries. Our binary evolution models, calculated with the MESA code, cover initial donor masses between $2.5 M_{\odot}$ and $10 M_{\odot}$ and initial accretor-to-donor mass ratios between $0.1$ and $1$. We find that the pre-instability evolution is qualitatively the same for all DDI donor stars, consisting of a long slow dimming phase followed by a shorter phase of rapid brightening. The latter phase is powered by recombination of hydrogen and accompanied by a strong increase in effective temperature, unique to unstable mass-transfer binaries. We estimate that a significant fraction of the rapid brighteners should be detectable by Gaia throughout the Galaxy. We model the progenitors of LRNe M31-2015 and V838 Mon, find a higher initial donor mass for M31 2015 than past estimates, and propose a new scenario for V838 Mon in which the known tertiary star dominates the pre-outburst photometry and the outburst results from the DDI of a more massive primary star. This work provides a comprehensive framework linking theory to observations of transients and enables improved classification and prediction of mass-transfer events.

Figures (12)

• Figure 1: Representative evolution of two $5\;M_{\odot}$ donor stars, one that will experience a DDI, and one that will not. The donor stars fill their Roche lobes at the same moment in their detached evolution (at a size of $15\;R_{\odot}$), but lose mass in different binary configurations. The DDI system, shown in red, contains an initially $1\;M_{\odot}$ companion star, whilst the stable mass transfer system, shown in blue, starts with an initially $1.5\;M_{\odot}$ companion. Points of interest are labelled with letters, which are explained in the text and Table \ref{['tab:points_of_interest']}. In the lower left panel, the dotted line shows the mass transfer rate based on the Kelvin-Helmholtz thermal timescale at point A.
• Figure 2: Structural changes in the donor star of a DDI binary system, shown for the same representative DDI system as in Figure \ref{['fig:HRD_labels']}: a donor star with a ZAMS mass of $5\;M_{\odot}$ that fills its Roche lobe at a size of $14.3\;R_{\odot}$ and loses mass to an initially $1\;M_{\odot}$ companion. From left to right, the panels show how the internal luminosity, specific energy generation due to H and He recombination and shortest timescale vary throughout the donor star during the mass transfer. For visual clarity, negative values (which have a minimum of about $-2\cdot 10^{6}$ ergs/s/g) are not shown in panel b. The total mass of the donor star $M_{\rm d}$ decreases from left to right along the horizontal axis. The vertical axis shows the amount of mass exterior to a shell at mass coordinate $m$, relative to the total donor mass. In this coordinate system, the core of the star lies towards the bottom of the figure, and the envelope towards the top. Lines of constant $\log\;\rho$ are drawn in white and dashed, with the corresponding values in cgs units indicated on the right hand side of each panel. The solid red line shows the location of the Roche lobe equivalent volume radius inside the star. The vertical green lines and associated letters correspond to the moments shown in Figure \ref{['fig:HRD_labels']} and described in Table \ref{['tab:points_of_interest']} and the text.
• Figure 3: Structural changes in the donor star of the same representative DDI system as in Figures \ref{['fig:HRD_labels']} and \ref{['fig:DDI_structure_2D']}. Panel a shows the structure of the donor star in the $\log \rho - \log T$ (in cgs units) plane at different moments in the evolution. Panels b and c show how, respectively, the specific entropy (per baryon) and internal luminosity vary throughout the donor star, using the fractional exterior mass as horizontal coordinate. In panel a, gray dashed lines correspond to adiabats. In panels b and c, gray shading indicates where the star is convective. In all panels, the lines are coloured and labelled according to the moments in the legend (see also Table \ref{['tab:points_of_interest']}). Crosses indicate the location of the Roche lobe equivalent volume radius inside the star.
• Figure 4: HRD locations of all our models at point B (the onset of RLOF; panel a), point D (the luminosity minimum; panel b) and point F (near-complete adiabatic stratification of the envelope; panel c). In all panels, the data points are coloured according to the corresponding initial donor mass and the marker type reflects the initial mass ratio, as indicated in the legend. Panel c furthermore shows the HRD tracks of the donor stars during the 10 years preceding point F.
• Figure 5: HRD evolution for our representative binary system (Figure \ref{['fig:HRD_labels']}) under different assumptions for the mass-transfer efficiency. All non-accreted matter is taken to leave the system with the specific AM of either the accretor (dashed lines) or a circumbinary ring with a radius of 1.2 times the semi-major axis (dotted lines). The line colours correspond to different mass-transfer efficiencies, as indicated in the legend.