Astronomers have made a groundbreaking discovery by finding TIC 120362137, a very small quadruple star system, which was recently published in Nature. This “3+1” star system is the most tightly bound one ever found. It gives us valuable information about how stars evolve and how orbits work. A team from the University of Szeged in Hungary led the study, which shows how stars move in a system that could fit between Jupiter and the Sun. Scientists can figure out how stars form and how stable they are over time by studying systems like these.
The Quadruple Star System TIC 120362137 Breaks Records
TIC 120362137 is a major breakthrough in the study of astronomy. Tamás Borkovits, the team leader and a researcher at the University of Szeged in Hungary, says that this system is the smallest known 3+1-type quadruple star system. The system has three stars that are very close together and orbit each other, as well as a fourth star that is farther away. The three stars in the middle are so close together that they could easily fit inside Mercury’s orbit, which is the planet closest to the Sun. This small structure goes against what we thought we knew about how big and how multi-star systems form. It also gives us new ways to study the forces that control these systems.
The top row shows the system from above, and the middle row shows it from the side, as TESS sees it. The bottom part shows the light curve that TESS saw and the light curve from the photodynamical model between July 22 and August 4, 2022. This is almost like a part of the Sector 54 observations of the TESS spacecraft. The whole quadruple star system is shown in the left columns, and the movement of the inner triple subsystem is shown in the right columns. Credit for the picture goes to Brian P. Powell at NASA Goddard Space Flight Center.
It wasn’t clear right away that this tight star system had been found. At first, the astronomers thought they were looking at a more common eclipsing binary system, which is two stars that eclipse each other every so often, causing small changes in brightness. Borkovits remembered,
“By just looking at the early TESS data, we could tell that TIC 120362137 is a compact, tight, triply eclipsing triple star system.”
At first the scientists saw two stars that blocked each other’s light every 3.3 Earth days, which made the stars dim for one to two hours at a time. In astronomy, these kinds of systems are not uncommon, so the team didn’t expect anything out of the ordinary at that point.
The Discovery of a Fourth Star
It wasn’t until more research that the system’s true complexity became clear. Borkovits went on,
“Then we figured out that there are extra fadings that last one to two days every 25 to 26 days. This made it clear that there must be a third star in the system with an orbital period of about 51 days.”
The researchers quickly discovered that TIC 120362137 was not merely a binary or even a triple system, but a triply eclipsing triple system, representing a notable divergence from conventional star systems.
Two common third-body eclipses of TIC 120362137 from TESS Sector 54. a
TESS saw three stars almost in a straight line because a third-body eclipse happened around a primary eclipse of the inner binary. This meant that the two inner stars eclipsed the more distant tertiary star at the same time, creating only one long-duration extra eclipse (superposed on the regular primary eclipse of the inner binary). During the next event, the tertiary star blocked out the two inner binary members one at a time. This made it possible to see two extra dips, the first of which started before the end of a regular primary eclipse of the inner pair. Also, keep in mind that the sharp, regular eclipses are the normal primary and secondary eclipses of the inner eclipsing binary. It is easy to tell that every other regular eclipse has a flat bottom, which means that it is a total (secondary) eclipse. The blue points show the observed data from TESS, which was binned in 1800-s intervals. The red smooth curves show the best-fit photodynamical solutions (see the Methods section on Spectro-photodynamical analysis). The bottom parts of both panels show the light curves that are left over. A few typical observational error bars are shown in black at the start of both residual curves. A Source data file contains the source data.
The team still hadn’t found the fourth star, though, even after this news. The Tillinghast Reflector Echelle Spectrograph (TRES) on the 1.5-meter Tillinghast telescope in Arizona was used to collect more data. It wasn’t until then that the existence of the distant fourth star was confirmed.
Borkovits said, “TIC 120362137 is a record-holder because we found that the outermost star has an orbital period of only about 1,046 days, which is the shortest of all the currently known 3+1 quadruple stars by a long shot.”
This makes TIC 120362137 the smallest 3+1 system found so far. Its outer star goes around the inner trio in just over three years.
The Difficulty of Finding Such Systems
Astronomers have a very hard time finding multi-star systems like TIC 120362137. Finding a fourth star is a rare event that takes a lot of time and careful observation. Borkovits said,
But finding systems like these is very, very hard. It takes a lot longer, maybe even decades or longer, to find a fourth, farthest component by looking at eclipses in the same way as the inner system.
Finding the fourth star is so hard that it often happens by chance, which makes finding TIC 120362137 even more amazing.
The Future of White Dwarfs: The Evolution of TIC 120362137
The team’s study, which was published in Nature, also looked at the system’s long-term future. They used computer simulations to figure out what would happen to the stars in TIC 120362137 in the end. Their research shows that the system will change into a binary system of white dwarfs.
“The biggest star, which is the main part of the innermost binary, will become a red giant first. In that state, it will join with its partner, the secondary star of the innermost binary. Borkovits said, “We call this daughter stellar body A’.”
This will start a chain of mergers that will end with the new star merging with the third star in about 276 million years.
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As the process goes on, the system will eventually break down into two white dwarfs, which are the remains of stars that can no longer fuse. Borkovits said, “Finally, our evolutionary model predicts the binary of these two white dwarfs with an orbital period of about 44 days.” This last state marks the end of the system’s stellar lifecycle. It gives us a look at what will happen to star systems like ours in the future.
