Most compact quadruple star system yet fits within an area the size of Jupiter’s orbit

Astronomers have reported observations of a rare star system consisting of one star orbiting a system of three more tightly bound stars. This quadruple star system is described in a new study, published in Nature Communications, as the most compact quadruple star system observed to date, with all four stars fitting into an area the size of Jupiter’s orbit.

3+1‑type quadruple star systems

Because the system consists of one star orbiting a 3-star subsystem, it is referred to as a 3+1‑type quadruple star system. This is opposed to a 2+2-type, which consists of two inner two-star subsystems, revolving around their combined center of mass with a much wider separation. Both types of systems are described as hierarchical. Understanding these systems can shed light on star formation, dynamics, and the evolution of exotic stellar objects.

Currently, only two other 3+1‑type quadruple star systems have been observed, as smaller, two-star (binary) systems appear to be far more common. The two known 3+1‑type systems were less compact compared to the recent discovery, which goes by the name “TIC 120362137.”

The study authors note, “It is very likely that there exist a number of other, similarly compact, 3 + 1 type quadruple stellar systems. Their discovery is, however, very difficult and may depend on only some occasional, fortuitous properties of these systems.”

Direct detections of all four stars

The team seems to have encountered some luck in finding this new system, as they say its brightness allowed for the ability to conduct both ground-based photometric and spectroscopic observations. This was combined with initial data from the Transiting Exoplanet Survey Satellite (TESS) satellite.

Although it was not immediately obvious this was a four-star system, these observations helped the astronomers find variations in the light that made it possible to discern the four separate stars and measure their properties directly. This direct spectroscopic detection is also a first for 3+1‑type quadruple star systems.

Their results showed that TIC 120362137’s inner triple system fits within an area the size of Mercury’s orbit, while the fourth star orbits within the distance of Jupiter from the sun. The team was also able to determine the temperatures and sizes of all four stars. They found that the three inner stars are more massive and hotter than the sun, although in varying degrees. The most massive star is the primary of the innermost binary (called star Aa). The fourth outer star is smaller, more similar in size and temperature to the sun and has a period of 1046 days. The periods of the inner stars are much shorter, ranging from a few days to 51 days.

The origin and fate of TIC 120362137

The team says that the inclination of the system is relatively flat. This gives some information about its initial formation.

“Such a flatness is very likely primordial, that is, a residual of the formation process of the entire quadruple system. The most likely scenario is that all four stars were formed from the very same, originally flat, disk with a sequential fragmentation process. Such a process, which favors the formation of nearly equal mass binary or multiple star components, and which had to be followed with some disk-driven migration, resulting in more compact inner subsystem(s) (as is the case in our current system),” the study authors explain.

The orbits of the stars within TIC 120362137 are currently stable, and the researchers say they should remain so throughout the main sequence lifetime of these stars. Simulations, however, indicate that eventually these stars will most likely collapse into a binary white dwarf system when the inner three stars merge. Yet, continued monitoring of these and other quadruple systems could reveal more about the dynamical evolution of these rare systems.

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