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When white dwarfs—the heated remnants of stars similar to our Sun—are closely orbited by another star, they occasionally siphon mass from their companion. The appropriated material accumulates on the surface of the white dwarf, instigating explosions known as novae.
Researchers have long inferred how these unstable alliances, termed cataclysmic variables (CVs), originate, but a recent study led by Caltech unveils an unexpected twist: In certain scenarios, a third star, revolving at a greater distance from the primary duo, may actually be the catalyst for the initial pairing of the star couple.
“Our findings are unveiling an alternative formation pathway for CVs,” explains Kareem El-Badry, assistant professor of astronomy at Caltech and co-author of a novel paper published in the Publications of the Astronomical Society of the Pacific. “Often, a lurking third star is crucial,” he notes. The primary author of the study is Caltech graduate student Cheyanne Shariat.
Previously, scientists assumed that CVs developed through a mechanism labeled common envelope evolution, whereby the partner stars draw closer together via a gas envelope surrounding them. An aging star on the path to becoming a white dwarf expands into a red giant that engulfs both stars, forming a shared envelope. This envelope confines the two stars, inducing them to spiral inwards. Eventually, the envelope is expelled, resulting in a compact pair that is close enough for the white dwarf to acquire its companion’s mass.
Although a third star wasn’t mentioned in these explanations, the research team speculated whether one might be involved. After all, they argued, the dynamics of triple-star systems do influence other types of star arrangements.
To delve deeper into the subject, the researchers utilized data from the European Space Agency’s Gaia mission, which is now retired. By analyzing these observations, they pinpointed 50 CVs located within hierarchical triple-star systems, or triples, as the researchers refer to them. A hierarchical triple is characterized by two stars being relatively close together, while the third is significantly distanced and orbits the primary duo. The findings indicated that at least 10 percent of all identified CVs belong to triple systems.
This 10 percent figure exceeded expectations if triples played no part in CV formation, prompting the researchers to investigate further through computer simulations. They conducted three-body simulations on 2,000 theoretical triples; these simulations accelerated the gravitational interactions among the trio of stars, advancing their evolution over time.
In the triple-star simulations, CVs emerged without the conventional method of common envelope evolution 20 percent of the time. In these instances, the researchers state, the third star influenced the primary binary.
“The gravity of the third star induces the binary stars to adopt a highly eccentric orbit, thereby pushing the companion star nearer to the white dwarf. Tidal forces expel energy and contract and circularize the orbit,” Shariat elaborates. “The star does not need to spiral in through the common envelope.”
In 60 percent of the simulations, the third star facilitated the initiation of the common envelope evolution process, drawing the two primary stars close enough to be enveloped in the same envelope. In the remaining 20 percent of the simulations, the CVs originated via the traditional common envelope evolution pathway requiring only two stars.
When the researchers considered a realistic star population within our galaxy, including CVs that are confirmed to have originated from just two stars, their theoretical models estimated that around 40 percent of all CVs evolve within triple systems. This is higher than the 10 percent they observed using Gaia because, in numerous instances, the third stars can be either difficult to detect or have detached from the CV.
Ultimately, the simulation results provided predictions concerning the types of triple-star systems that would be more likely to yield CVs. Specifically, these triple systems are anticipated to commence in more expansive configurations, where the tightly knit pair and the third star are separated by more than 100 astronomical units (an astronomical unit, or au, is the distance between the Sun and Earth).
Revisiting the Gaia data, the researchers found corroboration: The triples with CVs indeed displayed wider separations on average compared to typical systems.
“For the past half-century, researchers have utilized the spiral-in common-envelope evolution model to explicate CV formation,” notes El-Badry. “No one had previously recognized that this was predominantly occurring in triples!”
The investigation titled “Cataclysmic Variables in Triples: Formation Models and New Discoveries” received funding from the Joshua and Beth Friedman Foundation Fund, NASA, the National Science Foundation, and Howard and Astrid Preston. The endeavor was conducted in collaboration with Smadar Naoz, a researcher at UCLA specializing in theoretical studies of triples. Additional authors include Antonio Rodriguez, a Caltech graduate student, and Jan van Roestel from the University of Amsterdam.
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