“One of a Kind” Massive Triple Star System Detected

Artist’s interpretation of HD 98800, a quadruple-star system located 150 light-years away in the constellation TW Hydrae. Bin Liu and Alejandro Vigna-Gomez suggest that the more massive tertiary-star system TIC 470710327 could have started in a similar configuration – two binary systems with one of them eventually merging into one, bigger star. TIC 470710327 is located very close to “Cassiopeia”. Credit: NASA/JPL-Caltech/UCLA

Compact, massive triple star system detected by University of Copenhagen researchers.

Earlier this year, researchers revealed the discovery of an extraordinarily compact “one-of-a-kind” system of three stars. A partnership between two young researchers at the Niels Bohr Institute at the University of Copenhagen is currently focusing on how this unusual combination of a binary set of stars and a revolving larger star can form.

Tertiary star formation on a massive scale

The star system is made up of a binary set of stars, two stars that orbit each other, and one more massive star that orbits the binary.

“As far as we know, it is the first of its kind ever detected”, Alejandro Vigna-Gomez says. “We know of many tertiary star systems (three star systems), but they are typically significantly less massive. The massive stars in this triple are very close together – it is a compact system. The orbital period of the binary (~1 d) is the same as that of the rotation of Earth (1 day). The combined mass of the two of them is twelve times the mass of our Sun – so rather big stars. The tertiary star is approximately 16 times the mass of our Sun, so even bigger! The inner orbit is circular in shape with close to six revolutions of the tertiary star around the binary per year. Pretty fast, when you consider the size of them – unsurprisingly, the system is very luminous, so at first they were detected as a stellar binary”.

Alejandro Vigna-Gomez from Mexico and Bin Liu from China in front of the blackboard in the old auditorium A at Blegdamsvej. Credit: Ola J. Joensen/NBI

The dedication of amateurs led to the discovery of the star system

The initial discovery of the unusual nature of this highly peculiar system was equally remarkable in that it was discovered by amateur astronomers.

A group of amateur astronomers who were combing through a public data set from NASA’s TESS observatory (Transiting Exoplanet Survey Satellite) discovered something unusual.

The level of professionalism can be quite high among amateurs, and they made the professional astronomers aware of anomalies in the detection. It was discovered that what had previously been thought to be two stars were really three. These kinds of communities are known as citizen scientists.

The combination of expertise in dynamics and stellar evolution proved to be fruitful

Postdoc Alejandro Vigna-Gomez explains that when investigating the formation of the star system, several options were considered.

• If, for instance, the bigger star formed first, it would likely have ejected material that would have disrupted the formation of a binary that close.
• Another possibility is that the binary and the third star formed separately from each other and eventually encountered and locked in their orbits because of gravity.
• – or a third possibility, where two binaries formed and one merged into one, bigger star.

This is where the combined efforts of Bin Liu, an expert on dynamics, and Alejandro Vigna-Gomez’s knowledge of star formation came in handy. They coded the options and ran more than 100.000 iterations on the computer, in order to assess the most likely outcome of this scenario.

The result turned out to be in favor of the two binary systems forming initially and one of them merging with one star. Their results show that the two binary scenario could in fact explain the origin of the triple system.

The way forward requires the expertise of an astronomer specializing in observations

“Now we have a model of the most likely scenario on this unique system. But a model is not enough. And there are two ways in which we can prove or disentangle our theory on this formation”, postdoc Alejandro explains. ”One is studying the system in detail and the other is to make statistical analysis on a population of stars. If we go into the system in detail, we shall have to rely on the expertise of an astronomer. We already have some preliminary observations, but we still need to go through the data, and make sure we are interpreting it well”.

Bin Liu continues: “We also encourage people in the scientific community to look at the data deeply”, Postdoc Bin Liu says. “Maybe there are more compact systems buried in the data. What we really want to know is whether this kind of system is common in our universe”.

Alejandro Vigna-Gomez and Bin Liu have now mapped out quite a lot of work for themselves. “We need to find an adequate telescope with available observing time somewhere in the world and talk to someone who is really an expert from the observational point of view. The data from telescopes needs to be understood correctly. What you see in a telescope is not entirely what you get, so to say. It must be interpreted”.

So more collaboration is underway, very much in the spirit of Niels Bohr, Alejandro says:

“A Chinese and a Mexican with each their different fields, dynamics, and star formation, meet at the Niels Bohr Institute and are now on the way to yet more scientific collaboration”.

Reference: “Mergers prompted by dynamics in compact, multiple-star systems: a stellar-reduction case for the massive triple TIC 470710327” by Alejandro Vigna-Gómez, Bin Liu, David R Aguilera-Dena, Evgeni Grishin, Enrico Ramirez-Ruiz and Melinda Soares-Furtado, 29 June 2022, Monthly Notices of the Royal Astronomical Society Letters.
DOI: 10.1093/mnrasl/slac067