Triple Stellar Systems as Gravitational Wave Sources

Ground-based gravitational wave detectors like LIGO and Virgo have brought significant attention to binary systems composed of black holes and neutron stars as gravitational wave sources. However, two white dwarfs in a binary system are expected to be far more numerous. In particular, the pre-merger phase of double white dwarfs could lead to high-energy astrophysical events that would emit gravitational waves detectable by the European Space Agency’s upcoming Laser Interferometer Space Antenna (LISA) mission. Understanding how these double white dwarfs form is essential to interpreting the future LISA data. For the first time, researchers at the Max Planck Institute for Astrophysics (MPA) have now quantitatively assessed the impact of triple evolution on LISA sources. This study underscores the importance of triple interactions in the formation of double white dwarfs, revealing previously unexplored pathways that contribute to the gravitational-wave sources LISA will observe.
 

Stars often form in hierarchical triples, where a close binary system is orbited by a distant third star. These triple systems undergo complex gravitational interactions, which can dramatically alter the evolution of the stars. Such interactions can induce mass exchange between stars, mergers, or the disruption of one of the stars, all of which influence the final configuration of the system. Thus, triple dynamics can play a pivotal role in driving white dwarf binaries into the gravitational wave frequency range detectable by LISA.

In this research, doctoral student Abinaya Swaruba Rajamuthukumar, along with a group of MPA researchers, studied how triple star systems contribute to the population of double white dwarfs detectable by LISA. They combined simulations of triple star evolution using the Multiple Stellar Evolution (MSE) code with a Milky Way-like galaxy from the cosmological simulation TNG50. The study found that approximately 7.2 million double white dwarfs emitting gravitational waves in the LISA frequency band originate from triple systems, nearly double the number formed in isolated binaries, which account for about 3.8 million. Moreover, about 57% of the LISA double white dwarfs from triples retain a bound third star, though it is typically too distant to leave an observable imprint on the gravitational wave signal.

The team identified five key evolutionary pathways through which triple systems can produce LISA-detectable sources. These include induced mass transfer, outer binary mergers, ejected tertiaries, triple common envelope phases, and effectively isolated inner binaries (see graphic). The overall population properties of double white dwarfs from triple systems and those with a binary-origin are largely indistinguishable. Interestingly, the triple channel introduces a rare but intriguing subset of highly eccentric systems that emit burst-like gravitational wave signals, offering a distinct observational signature for LISA.

This study provides the first detailed exploration of triple-star evolution in the context of gravitational wave astrophysics. As LISA prepares for launch in 2035, these findings will be essential for accurately interpreting the Galactic population of gravitational wave sources and refining data analysis techniques. The results underscore the need to account for triple evolution when modeling LISA sources, paving the way for a more comprehensive understanding of the Milky Way’s gravitational wave sources.