New Insights into Cataclysmic Variables: The Role of Third Stars

Astronomers have unveiled a groundbreaking discovery regarding the formation of certain explosive star systems, known as cataclysmic variables (CVs), which are characterized by white dwarfs stealing mass from companion stars. The study, led by researchers from the California Institute of Technology (Caltech), reveals that a third star in some systems may play a crucial role in the formation of these cataclysmic events. The findings, published in the *Publications of the Astronomical Society of the Pacific* on July 9, 2025, challenge previously held beliefs about the evolutionary pathways of these stellar phenomena.

In traditional models, CVs were understood to form through a process called common envelope evolution. In this scenario, an aging star expands and engulfs its companion, creating a shared gas envelope that facilitates the stars spiraling closer together. Eventually, the envelope is expelled, allowing the white dwarf to begin mass transfer from its partner, triggering explosive novae. However, the new research indicates that a third star, orbiting at a greater distance, may significantly influence this process by altering the dynamics of the binary pair.

Kareem El-Badry, an assistant professor of astronomy at Caltech and co-author of the study, explains, "Our results are revealing another formation channel for CVs. Sometimes, a lurking third star is key." The lead author, Cheyanne Shariat, a graduate student at Caltech, added that after analyzing data from the European Space Agency's Gaia mission, the team identified 50 CVs in hierarchical triple-star systems. Their analysis suggests that approximately 10% of all known CVs are part of these triple systems, a figure higher than previously expected.

To further explore the implications of these findings, the researchers conducted simulations of 2,000 hypothetical triple-star systems. Remarkably, they discovered that in 20% of these simulations, CVs formed without the traditional common envelope evolution mechanism. Instead, the gravitational influence of the third star resulted in increased eccentricity in the binary orbit, bringing the companion star closer to the white dwarf without the need for a shared envelope. Shariat stated, "The gravity of the third star causes the binary stars to have a super-eccentric orbit, forcing the companion star closer to the white dwarf."

In 60% of the simulations, the presence of a third star initiated the common envelope process, aligning with the traditional model but demonstrating an additional pathway for CV formation. When the researchers simulated a realistic population of stars in the Milky Way, they predicted that around 40% of CVs could form in triple systems. This percentage is notably higher than the observed 10%, indicating that many third stars may be difficult to detect or could have become unbound from their binary partners.

The findings have significant implications for our understanding of stellar evolution and the formation of explosive phenomena in the universe. The research emphasizes the importance of considering multi-star systems in astrophysical studies. El-Badry concluded, "For the past 50 years, people were using the spiral-in common-envelope evolution model to explain CV formation. Nobody had noticed before that this was largely happening in triples."

As astronomers continue to refine their understanding of cataclysmic variable systems, this research not only challenges established models but also opens new avenues for exploration in the field of stellar dynamics. The implications of these findings extend beyond mere academic interest; they potentially reshape our comprehension of star formation and life cycles across the universe, highlighting the intricate relationships between multiple stars and their evolutionary paths.