Using NASA's exoplanet-hunting spacecraft TESS (Transiting Exoplanet Survey Satellite) astronomers have discovered an extraordinary quadruple star. The system is the tightest 3+1-star system, a subset of quadruple star systems, yet discovered. Excitingly, the discoverers of this system were also able to determine what its final fate will be.
The system TIC 120362137 consists of a stable and tightly bound inner system of three stars orbiting each other that are orbited by a more distant outer star observing the system from afar. While the outer star is located at around the same distance from the stellar triplet as the distance from Jupiter to the sun, the inner stellar sub-system would fit within the orbit of Mercury, the closest planet to the sun, around our star.
TIC 120362137 is an important discovery for researchers because, in addition to 3+1 systems being extremely rare — as a so-called hierarchical star system, where several stars orbit each other within a relatively small area — TIC 120362137 could also help us better understand stellar formation and long‑term orbital stability.
"TIC 120362137 is currently the most compact known 3+1-type quadruple star system," team leader Tamás Borkovits, a researcher at the University of Szeged, Hungary, told Space.com.
The extraordinary nature of this system wasn't immediately obvious, however.
"By a simple inspection of the early TESS data, we realized that TIC 120362137 is a compact, tight, triply eclipsing triple star system," Borkovits said. The researcher added that when the team first saw TIC 120362137, the hitherto unknown system initially seemed to consist of a pair of stars eclipsing each other every 3.3 Earth-days, creating a drop in brightness lasting between one and two hours.
"We know thousands of such systems, called eclipsing binaries. Therefore, there was nothing interesting or peculiar at that stage," he continued. "Then, we realized that there are extra one-to-two-day-long fadings every 25 to 26 days, which made it clear that there must be a third star also in the system, with an orbital period of around 51 days. Therefore, we found that TIC 120362137 must be a triply eclipsing triple system.
"However, we still did not know about the fourth star at that moment."
The team then saw further eclipses, indicating a fourth star, the presence of which was confirmed using the Tillinghast Reflector Echelle Spectrograph (TRES) on the 1.5-meter Tillinghast telescope located on Mt. Hopkins in Arizona.
"TIC 120362137 is a record-holder in the sense that we found that the outermost star has an orbital period of only around 1,046 days, which is the shortest amongst all the currently known 3+1 quadruple stars by far," Borkovits said. "The discovery of such systems, however, is very, very difficult. To discover a fourth, most distant component by checking eclipses in the same way as the inner system requires much more time, maybe even several decades or longer. Other kinds of detection of a fourth star may happen, but only serendipitously."
The team was able to determine other characteristics of the stars in this system, too. The scientists found the three innermost stars are more massive and hotter than the sun, while the outermost component, the fourth star, is cooler, less massive and thus similar to the sun. Additionally, by using computer simulations, the researchers were able to determine the future of this 3+1 star system, ending up as just two white dwarf stellar remnants.
"First, the most massive star, which is the primary component of the innermost binary, will reach the red giant state. In that state, it will merge with its mate, the secondary star of the innermost binary. We call this daughter stellar body A'," Borkovits said. "Then, in around 276 million years, in a second step, this new, merged star A' will merge with the third stellar component, star B, when both stars have reached the red giant stage. We call this massive new star AB."
He added that, following this, the star AB will lose a significant part of its mass, eventually collapsing to form a white dwarf. As this happens, the distant fourth star will undergo a similar process, creating the second white dwarf.
"Finally, therefore, our evolutionary model predicts the binary of these two white dwarfs with an orbital period of around 44 days," Borkovits said. "The more massive white dwarf with a mass of around 89% the mass of the sun is formed after two mergers involving the three inner stars, while the less massive white dwarf, with a mass around 29% that of the sun, is simply formed from the fourth, most distant star.
The team's results were published on Tuesday (March 3) in the journal Nature.
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