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. Interestingly, 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 internal system of wood stars orbiting each other orbiting a more distant outer star observing the system from a distance. While the outer star is at approximately the same distance from the star triplet as the distance from Jupiter to sunwould the inner stellar subsystem fit within the orbit of Mercurythe closest planet to the sun, around our star.
TIC 120362137 is an important discovery for scientists because, in addition to the fact that 3+1 systems are extremely rare – such as a so-called hierarchical star system, where several stars orbit each other within a relatively small area – TIC 120362137 can also help us better understand star 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.
However, the extraordinary nature of this system was not immediately obvious.
“On a simple inspection of early TESS data, we realized that TIC 120362137 is a compact, dense, triple-eclipsing triple star system,” said Borkovits. The scientist added that when the team first saw TIC 120362137, the previously unknown system initially appeared to consist of a pair of stars that eclipsed each other every 3 days and created a drop on Earth every 3 hours.
“We know thousands of such systems, called eclipsing binaries. So there was nothing interesting or peculiar at that stage,” he continued. “Then we realized that there are extra one- to two-day-long fades every 25 to 26 days, which made it clear that there must be a third star in the system as well, with an orbital period of about 51 days. Therefore, we found that TIC 120362137 must be a triple-eclipsing system.
“But we still didn’t know about the fourth star at that moment.”
The team then saw additional eclipses, indicating a fourth star, whose presence 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 about 1046 days, which is the shortest of all the currently known 3+1 quadruple stars,” Borkovits said. “However, the discovery of such systems is very, very difficult. Detecting a fourth, most distant component by checking eclipses in the same way as the inner system requires much more time, perhaps even decades or longer. Other types of detection of a fourth star may happen, but only serendipitously.”
The team was also able to determine other properties of the stars in this system. The researchers found that 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, using computer simulations, scientists were able to determine the future of this 3+1 star system, ending up as only two white dwarf star 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 companion, the secondary star of the innermost binary. We call this daughter stellar body A’,” Borkovits said. “Then, in about 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 after this the star AB will lose a significant part of its mass, eventually collapsing to form a white dwarf. When this happens, the distant fourth star will undergo a similar process, creating the second white dwarf.
“In the end, therefore, our evolutionary model predicts the binary of these two white dwarfs with an orbital period of about 44 days,” Borkovits said. “The more massive white dwarf with a mass of about 89% of the mass of the Sun is formed after two mergers involving the three inner stars, while the less massive white dwarf, with a mass of about 29% of the Sun, is simply formed from the fourth, most distant star.
The team’s results were published Tuesday (March 3) in the journal Nature.
