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Close Encounters of Distant Stellar Partners

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V838 Monocerotis Hubble image
V838 Monocerotis, shown at the center of this Hubble Space Telescope image, was the site of a luminous red nova outburst that may have been the result of two stars colliding. [NASA, the Hubble Heritage Team (AURA/STScI) and ESA; CC BY 4.0]

Stars orbiting one another on extremely wide orbits seem destined to remain apart forever — but new research shows that the subtle influence of the Milky Way’s gravitational pull can bring them crashing together.

Loosely Connected Stars

Many stars in our galaxy travel through space with a binary partner, but some binary pairs are closer than others. Surveys suggest that as many as 12% of nearby Sun-like stars are members of wide binary systems with separations of 100 to 100,000 au or more. Unlike close stellar binaries that might influence each other’s evolution through accretion, stellar winds, and radiation, wide binaries are expected to evolve as if their stellar partner didn’t exist.

Though unaffected by their partners, stars in loosely connected binary systems can be swayed by the subtle influence of the Milky Way’s background gravitational pull, or they can be knocked off course by passing stars. How do these factors affect the evolution of wide binary systems?

Unexpectedly Close

A team led by Jakob Stegmann (Max Planck Institute for Astrophysics) used relativistic N-body simulations to study how the background gravitational pull of the Milky Way and nudges from passing stars affect the orbits and evolution of stars in wide binary systems.

Plot showing outcomes for wide binaries with different separations and distances from the galactic center

Typical outcomes for wide binaries as a function of initial binary separation and distance from the galactic center. Click to enlarge. [Stegmann et al. 2024]

The team simulated 100,000 binary systems with varying initial separations and allowed them to evolve for as long as 14 billion years. The outcome depended on the initial separation of the binary. Binaries orbiting within 100 au of one another weren’t much affected by the galactic tide or by stars whizzing by. Wide binaries with separations of 100,000 au (about 1.6 light-years) or greater were swept up in the galactic tide and torn apart. In between these two extremes, though, the binaries had the potential to end up closer than before.

Swept Up in the Galactic Tide

Many of the binaries underwent wild eccentricity oscillations that brought their orbits from nearly circular to narrow and elongated. This means that initially widely separated stars can undergo close encounters, mass-transfer events, and even mergers.

plot showing the evolution of a widely spaced binary system's separation over time

An example of the evolution of a binary’s separation. This particular binary collided after 2.3 billion years, producing gravitational waves. Click to enlarge. [Adapted from Stegmann et al. 2024]

The precise outcome depended on the masses of the stars involved. High-mass stars evolved into neutron stars or black holes long before the slow eccentricity evolution brought them close, leading to compact-object mergers. The authors estimated that 0.4–5% of observed binary black hole gravitational-wave events could be due to mergers brought about this way. Slightly less massive stars evolved into white dwarfs before merging, generating Type Ia supernovae, though this process likely accounts for just 0.01–0.1% of observed Type Ia supernovae. Low-mass stars interacted while they were still on the main sequence, leading to divergent outcomes: they either tidally interacted and were drawn into close, stable orbits, or they collided, producing luminous red novae.

There are more intricacies to the model results than can be covered in this brief highlight, so be sure to check out the full research article for all the details!

Citation

“Close Encounters of Wide Binaries Induced by the Galactic Tide: Implications for Stellar Mergers and Gravitational-Wave Sources,” Jakob Stegmann et al 2024 ApJL 972 L19. doi:10.3847/2041-8213/ad70bb