Binary imposters: Mergers in massive hierarchical triple stars

TL;DR

This work shows that mergers in massive hierarchical triples are a common evolutionary outcome, occurring in roughly 20–32% of systems. Using a coupled triple-population synthesis with TRES and SeBa, the authors quantify the properties of post-merger binaries, their diverse evolutionary channels, and the implications for observables such as age discrepancies in wide MS+MS binaries, HMXBs, and gravitational-wave progenitors. The majority of post-merger systems become wide, eccentric binaries that often avoid further interaction, while a substantial minority undergo additional mass transfer or lead to compact-object formation, enabling GW mergers. The results indicate that triple evolution substantially contributes to the massive-star binary population and to the diversity of end states, including HMXBs and GW sources, with Galactic merger rates of order 10^-3 yr^-1. The work also underscores observational pathways to identify past mergers via rejuvenation signatures and non-coeval binaries, albeit with caveats stemming from model simplifications and uncertainties in mass-transfer and merger physics.

Abstract

Massive stars are often born in triples, where gravitational dynamics and stellar interactions play a crucial role in shaping their evolution. One such pathway includes the merger of the inner binary, transforming the system to a binary with a distinct formation history. Therefore, the interpretation of observed binary properties and their inferred formation history may require the consideration of a potential triple origin. We aim to investigate the population of stellar mergers in massive hierarchical triples. Specifically, we assess how frequently mergers occur, and characterise the properties of the post-merger binaries and their subsequent evolution. We combine the triple population synthesis code TRES, which self-consistently models stellar evolution, binary interaction, and gravitational dynamics, with the binary population synthesis code SeBa to simulate 10^5 dynamically stable, massive triples from the zero-age main sequence through merger and post-merger evolution. We explore the effects of a range of physical models for the initial stellar properties, mass transfer, and merger. We find that stellar mergers are a common outcome, occurring in 20-32% of massive triples. Most mergers happen relatively early in the evolution of the system and involve two main-sequence (MS) stars, producing rejuvenated merger remnants that can appear significantly younger than their tertiary companions. Consequently, we predict that 2-10% of all wide MS+MS binaries (P>100 days) have a measurable age discrepancy, and serve as a promising way to identify merged stars. The post-merger systems preferentially evolve into wide, eccentric binaries, with ~80% avoiding further interaction. However, a notable fraction (16-22%) undergoes a second mass-transfer phase, which may result in the formation of high-mass X-ray binaries or mergers of compact objects that spiral in via gravitational-wave emission.

Figures (8)

• Figure 1: Distributions of the initial (ZAMS) orbital properties for systems that result in a stellar merger in the fiducial model. On the y-axis, we show the number of simulated systems. Properties of the inner orbits are shown in blue, outer orbits in red, and the complete simulated population is shown in gray for comparison. Panels show (top left) semi-major axis, (top right) eccentricity, (bottom left) mass ratio ($q = m_2/m_1$ for inner orbits, $q = m_3/(m_1+m_2)$ for outer), and (bottom right) mutual inclination between the inner and outer orbits.
• Figure 2: Distribution of the evolutionary phases of donor stars at the onset of mass transfer for systems in the fiducial model. On the y-axis, we show the number of simulated systems. Each coloured bar corresponds to a different evolutionary phase: Main Sequence (MS), Hertzsprung Gap (HG), First Giant Branch (FGB), Core Helium Burning (CHeB), and Asymptotic Giant Branch (AGB). The hatched gray overlay indicates the fraction of systems in each category that result in a stellar merger. Error bars (shaded regions) represent the range of values for the stellar mergers across all model variations.
• Figure 3: Fraction of merging triple systems that have experienced a merger of the inner binary as function of time for the fiducial model (black solid line). The dashed lines represent the fraction of systems --- from the population that at some point in time experience a stellar merger of the inner binary --- that contain a bound triple (blue), a binary (red), or only single stars (green) at a given time.
• Figure 4: Shown are 2D histograms of the post-merger orbital parameters: log orbital period (in days) vs. eccentricity (top), log orbital period vs. tertiary-to-merger-remnant mass ratio $q_{\rm{pm}}$ (middle), and mass ratio vs. eccentricity (bottom). Colour indicates the number of simulated systems in each bin, and histograms on top and side axes show the marginal distributions.
• Figure 5: Illustration of the evolutionary channels of massive triples that experience a stellar merger with their respective incidences. The system initially forms as a stable hierarchical triple and undergoes a stellar merger within the inner binary as a result of mass transfer. The subsequent evolution of the post-merger binary proceeds along one of three possible channels, depending on the system's properties at the time of merger. Mass transfer is abbreviated as MT.