Astronomers are puzzled by the first-ever elusive triple star system

How could it have formed?
Loukia Papadopoulos
Artist’s interpretation of HD 98800.NASA/JPL-Caltech/UCLA

Earlier this year, astronomers announced the discovery of an unusually compact “one of a kind” system of three stars. It was a unique combination of a binary set of stars and a revolving bigger star that puzzled the scientists that observed it. 

The first of its kind ever detected

“As far as we know, it is the first of its kind ever detected”, astronomer and postdoc Alejandro Vigna-Gomez said in a statement by the Niels Bohr Institute. “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 discovery was actually made by citizen scientists who were going through a public data set from NASA's TESS observatory (Transiting Exoplanet Survey Satellite). They then alerted the professionals to what they believed was something out of the ordinary, and indeed it was.

The tertiary star is approximately 16 times the mass of our Sun and boasts an inner orbit in a circular shape. This orbit performs close to six revolutions of the tertiary star around the binary per year. 

Three options for how the system came to be 

The scientists then had the difficult task of trying to conceive of the circumstances that led to the star system. Three options were given:

  • 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.
Astronomers are puzzled by the first-ever elusive triple star system
Alejandro Vigna-Gomez from Mexico and Bin Liu from China in front of the blackboard in the old auditorium A at Blegdamsvej. Source: Ola J. Joensen

Vigna-Gomez joined forces with Bin Liu, an expert on dynamics, to code the options and ran more than 100.000 iterations in order to assess the most likely outcome. The analysis pointed to the two binary systems forming initially and one of them merging with the one star. 

“Now we have a model of the most likely scenario on this unique system. But a model is not enough," Vigna-Gomez said. "And there are two ways in which we can prove or disentangle our theory on this formation. One is studying the system in detail, and the other is to make statistical analysis on a population of stars."

Vigna-Gomez and Liu are now hoping to get more support in figuring out this celestial mystery. “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," explained Liu.

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