The triple system TIC 470710327 may have originated as a pair of binary systems

Artistic concept of gravitationally bound binary systems (Image NASA/JPL-Caltech/UCLA)
Artistic concept of gravitationally bound binary systems (Image NASA/JPL-Caltech/UCLA)

An article published in the journal “Monthly Notices of the Royal Astronomical Society Letters” offers a number of clues about the triple system TIC 470710327 being the result of a stellar merger. A team of researchers used the HERMES spectrograph mounted on the Mercator Telescope for follow-up observations of this system, which was discovered thanks to NASA’s TESS space telescope, a discovery that was announced in an article published in April 2022 in the journal “Monthly Notices of the Royal Astronomical Society”. Computer simulations led the researchers to conclude that originally there were two gravitationally-bound binary systems and in one of them, the two stars merged into one that ended up forming a triple system with the other two.

The identification of multiple systems consisting of massive stars is considered crucial to testing the models describing the possibilities of stellar formation and evolution. However, their brightness can be a problem because it makes it difficult to resolve the various stars that make up a multiple system. For a long time, the TIC 470710327 system was considered binary and only recently the images captured by the TESS space telescope led to more in-depth examinations that allowed to understand its real nature.

In recent years, various scientific projects have been opened to the public with the possibility of contributing to various types of research. The Zooniverse platform includes various projects including PHT (Planet Hunters TESS), which allows citizen scientists to search for exoplanets. In the case of the TIC 470710327 system, in 2019, they reported anomalies that led to further examinations and the recognition that it’s a triple system. The search didn’t end there because it’s a system out of the ordinary even among multiple ones.

The three stars of TIC 470710327 form a very massive and compact system, much more massive than the known triple systems. Two stars orbit each other in almost exactly one Earth day and have an overall mass that is 12 times the Sun’s. The third star has a much higher mass since it’s about 16 times the Sun’s and orbits its two companions in about 52 Earth days. That’s a really bright trio and that’s why it took decades to understand its nature.

The configuration of the TIC 470710327 trio was examined to try to understand its origin. Computer simulations allowed to assess different hypotheses to understand which was more likely. One possibility was that the most massive star formed first but in that case, it would likely have ejected materials that would have interfered with the formation of the neighboring pair. Another possibility was that the more massive star had formed independently of the neighboring pair and only later approached them forming a gravitational bond.

The results of more than 100,000 simulations indicate that the most likely origin of the TIC 470710327 trio is in the formation of two gravitationally bound binary systems in which a pair merged to form the most massive star. According to the researchers, this is a scenario that could be common when several massive stars form in multiple systems, perhaps already during the star-forming phase if two of the protostars are close enough.

The conclusions reached by this study are reasonable but more evidence and therefore more follow-up observations are needed to confirm them. For example, the models predict that a stellar merger would lead to the birth of a highly-magnetized star with slow rotation. The next investigations could provide interesting results regarding the evolution of these multiple systems with ramifications concerning the evolution of massive stars, which have a short and intense life that ends with the ejection into interstellar space of many chemical elements important in the formation of new planets.

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