Formation of massive multiple-star systems: early migration and mergers

TL;DR

This work uses a high-resolution, Solar-metallicity star-cluster formation simulation that resolves binaries down to ~1 au to investigate how massive-star binaries assemble, migrate, and merge in their embedded phase. It identifies four primary formation channels—filament, disc, core fragmentation, and dynamical capture—and shows that rapid disc- and circumbinary-disc interactions drive inward migration, producing tight binaries with a_final often < 100 au. The results reveal a strong mass dependence of multiplicity, frequent mergers in massive systems, and isotropic inclinations consistent with turbulent formation; they also highlight the importance of disc-driven migration for tight massive binaries and discuss caveats from magnetic fields and resolution. Overall, the findings offer a cohesive, multi-scale picture of how massive stars form, evolve into close binaries, and potentially become progenitors of compact-object binaries and other high-energy transients.

Abstract

Massive stars are often found in multiple systems, yet how binary-star systems with very close separations ($\lesssim$ au) assemble remains unresolved. We investigate the formation and inward migration of massive-star binaries in Solar-metallicity environments using the star-cluster formation simulation of Chon et al. (2024), which forms a $1200\,M_\odot$ stellar cluster and resolves binaries down to 1 au separation. Our results indicate that stars more massive than $2\,M_{\odot}$ predominantly assemble in binary or triple configurations, in agreement with observations, with member stars forming nearly coevally. In most of these systems, the inner binary hardens by one to three orders of magnitude and reaches a steady-state within the first $0.1\,$Myr. Notably, all binaries whose final separations are below 10 au are hardened with the aid of circumbinary discs, highlighting disc-driven migration as a key to produce tight massive binaries. We further find that binaries form with random inclinations relative to the initial rotation axis of the cloud, and that mutual inclinations in triple systems follow an isotropic distribution, implying that stochastic interactions driven by turbulence and few-body dynamics are crucial during assembly and migration. Finally, stars with $M>2\,M_{\odot}$ often undergo repeated merger events during cluster evolution, yielding extreme mass ratios ($q<0.1$). Some of these products may evolve into compact-object binaries containing a black hole or neutron star, including X-ray binaries and systems detectable by Gaia.

Figures (22)

• Figure 1: Mass spectrum obtained from the base simulation. The red thick line shows the spectrum from the original data and the green thin line shows the distribution of the "merger-corrected" sample described in Section \ref{['sec:merger_correction']}. The dashed line shows the Salpeter IMF with $\mathrm{d}N/\mathrm{d}M_* \propto M_*^{-2.3}$1955ApJ...121..161S.
• Figure 2: Snapshot of the base simulation, showcasing examples of the key mechanisms driving multiple formation. Central panel: projected density distribution for the entire cloud scale at $t\approx0.56~$Myr. Panels a-d: zoom-in view of the binary progenitor clouds for four different binary formation channels, filament fragmentation (a), disc fragmentation (b), core fragmentation (c), and dynamical capture (d). The white squares a-b in the central panel indicate the regions where binary formation occurs. We do not specify the region of the binary formation by dynamical capture (panel d) because it occurs at $t\approx1.7~$Myr, far later than the shown snapshot. A scale bar representing $1$ pc (or $10^3$ au) is shown at the bottom-left part of the central panel (or panels a-d). Note that the colour-scale is different in each panel.
• Figure 3: Examples of the binary formation and evolution for different binary formation channels, filament fragmentation (A), disc fragmentation (B), and core fragmentation (C). We show the projected density distributions of the binary progenitor clouds. The time origin indicates when the fragmentation occurs. We denotes the member of the binary system or its progenitor cores by dashed circles.
• Figure 4: The number of surviving isolated and inner binary systems as a function of primary stellar mass is shown for (a) all binaries, (b) binaries with final separations smaller than $100~\mathrm{au}$, and (c) binaries with final separations larger than $100~\mathrm{au}$. Different colour bins indicate distinct binary formation channels: orange for filament fragmentation, blue for disc fragmentation, green for core fragmentation, and purple for dynamical capture.
• Figure 5: Time evolution of the orbital separation ($a_{\rm sep}$), measured from the binary's formation epoch. The time interval during which the binary interacts with a disc is highlighted in red. The system’s spatial resolution---calculated as the sum of the sink radii of the binary---is indicated by a dashed blue line. If the binary has a tertiary companion, its separation is shown as a solid green line. Merger events between sink particles are marked by thin solid grey vertical lines. Result for our base simulation at Solar metallicity with feedback included.