The Hidden Efficiency of Stellar Interactions
When two stars orbit close together, one star can transfer material to its companion, dramatically changing both stars' evolution. However, how much of this transferred material actually stays with the receiving star has remained one of the biggest mysteries in binary star physics. Using a new sample of 16 carefully studied binary systems, MPA scientists have now discovered that binary stars are much more efficient at keeping transferred material than previously thought, with many systems retaining more than half of the mass that was donated. This finding challenges decades of theoretical assumptions and has profound implications for our understanding of stellar evolution, affecting everything from the types of supernovae we observe to the formation of gravitational wave sources, X-ray binaries, and exotic stellar objects like blue stragglers.
Most stars in the Universe are born in binary or multiple star systems, where two or more stars orbit around their common center of mass. When these stars orbit close enough together, over the course of their lifetimes, they can interact gravitationally and exchange material. This can dramatically alter the evolution of both stars, leading to exotic stellar objects, different types of supernovae, and the formation of compact objects like neutron stars and black holes. Therefore, binary interactions play a key role in shaping the stellar populations we observe.
The research team focused on a special type of binary system called Be+sdOB binaries, which consist of a "stripped" star that has lost its outer layers, and a rapidly rotating star that was spun up by accreting these outer layers (see Figure 1). These systems are particularly valuable for studying mass transfer because they represent clear examples of past binary interaction. The stripped star reveals how much mass was originally donated, while the other star shows how much was actually retained.
Previous studies have successfully measured the masses of both stars in 16 such systems using a combination of state-of-the-art observational techniques. Namely, high-resolution interferometry from the CHARA Array and VLTI/GRAVITY instruments creates a powerful virtual telescope by combining light from multiple telescopes, allowing them to measure the tiny separations and orbital motions of close binary stars. These interferometric measurements, combined with detailed spectroscopic observations, enabled precise mass determinations. By comparing these present-day masses with stellar evolution models, the team at MPA could determine how much mass must have been transferred and retained during the binary interaction.
The results are striking: half of the systems require that more than 50% of the transferred mass was retained by the receiving star. This is in stark contrast to theoretical models that assume only a few percent of transferred material can be kept, based on the idea that rapidly rotating stars cannot accept much additional mass due to centrifugal forces (see Figure 2).
The most likely explanation for this efficient mass transfer is that accretion disks around the receiving star can carry away angular momentum while allowing matter to fall onto the star. This process, well-known in other astrophysical contexts, appears to be much more important in binary star evolution than previously recognized.
These findings will force a major revision of binary evolution models and have wide-ranging implications. Many high-profile theoretical predictions about stellar evolution rely on the assumption that mass transfer is highly non-conservative, which these findings are in strong tension with. The results suggest that mass-gaining stars will be much more massive than currently predicted, leading to different populations of supernovae, white dwarfs, and gravitational wave sources. The orbital properties of post-interaction binaries will also be affected, which provides important constraints for understanding the formation of exotic stellar objects.
