Tricky Triplets: How Simulations Reveal New Paths for Massive Triple Stars
Triple star systems, where three stars orbit one another, are more common among the most massive stars than previously thought. In their new study, Luca Sciarini and collaborators explore how these massive trios evolve over time. Specifically, they look at how the details of stellar modeling—such as how stars expand or lose mass through winds—can drastically change the predicted fates of these systems. Using a new approach that combines realistic stellar evolution modeling with the complex dynamics of triple systems, the team highlights how traditional models may miss crucial features of triple star evolution.
Simulating Triple Star Evolution
The authors use a simulation code called tres to study triple star systems. This code tracks the motion and interaction of the three stars over time, and it typically relies on a fast (but approximate) model called seba to describe how individual stars evolve. However, these quick models make simplifications that don’t hold up well for very massive stars. To improve accuracy, Sciarini’s team replaces seba with a more detailed and physically accurate model called mesa. By integrating mesa directly into the triple system simulations, they can model how the stars change internally and externally in real time.
Why It Matters: Stellar Physics Impacts Star Fates
One key aspect of a star’s evolution is how large it gets (its radial expansion) and how much mass it loses through stellar winds. These factors influence whether stars in a triple system will get close enough to interact—transferring mass, merging, or even causing one star to be ejected. The team compares predictions from the old seba models to those from the new mesa-based approach and finds large differences for stars heavier than 50 times the mass of the Sun. For example, mesa predicts significantly smaller star sizes and stronger effects from mass loss, leading to fewer interactions in some cases and more in others.
Single Stars, Big Differences
Before testing full triple systems, the authors compared how mesa and seba predict the evolution of single massive stars. They found that for stars heavier than 50 solar masses, mesa and seba give very different results—especially regarding how big stars get and how long they stay in certain evolutionary stages. These differences are mainly due to how seba approximates the effects of mass loss, which can dramatically change how stars expand or shrink over time.
Dramatic Consequences in Triple Systems
When these modeling differences are applied to triple systems, the outcomes diverge. In one case, using seba predicts that the outer star in the system transfers mass to the inner binary, while mesa predicts that the inner two stars interact first. In another example, seba suggests the stars merge peacefully, while mesa shows the system becoming unstable and breaking apart. These aren’t just small differences—they could affect how astronomers understand the origins of black holes or neutron star collisions observed via gravitational waves.
Eta Carinae: A Test Case
To check the accuracy of their models, the team simulated the famous massive binary system Eta Carinae, believed to have once been part of a triple system. Both mesa and seba predict the stars eventually merged, supporting the idea that Eta Carinae's dramatic past could be explained through triple star dynamics. However, mesa shows that more subtle processes—like how a star’s internal structure affects its gravitational pull—can make ZLK oscillations (a kind of orbital wobble) last longer, which in turn could affect when and how stars in the system interact.
A Call for Caution in Predictions
The takeaway from this study is that the method used to model stellar evolution really matters, especially for very massive stars in complex systems. The team urges caution when relying on fast models like seba, especially for stars more massive than 50 solar masses or when strong stellar winds are expected. Their work opens up new possibilities for accurately predicting how massive triple systems evolve and ultimately contribute to cosmic events like supernovae or gravitational wave mergers.
Source: Sciarini
