TIC 470710327, a massive, compact hierarchical triple-star system recently identified by NASA’s Transiting Exoplanet Survey Satellite (TESS), was originally a 2+2 quadruple-star system, according to new research led by Niels Bohr Institute astronomers.
An artist’s impression of the HR 6819 triple system. Image credit: L. Calçada / ESO.
TIC 470710327 was identified earlier this year in TESS data and followed up with the HERMES spectrograph on the 1.2-m Mercator Telescope at the Roque de Los Muchachos Observatory on La Palma.
Located in the constellation of Cassiopeia, this system is comprised of a circular binary system and a massive third companion on a 52-day orbit.
“As far as we know, it is the first of its kind ever detected,” said Dr. Alejandro Vigna-Gomez, an astronomer with the Niels Bohr Institute at the University of Copenhagen.
“The orbital period of the binary system is the same as that of the rotation of Earth (one day).”
“The combined mass of the two of them is 12 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.”
Astronomers know of many triple-star systems, but they are typically significantly less massive.
The massive stars in the TIC 470710327 system are very close together — it is a compact system.
“When investigating the formation of the star system, several options were considered,” Dr. Vigna-Gomez said.
“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.”
In order to assess the most likely formation scenario, the astronomers coded the options and ran more than 100,000 iterations on the computer.
Their result turned out to be in favor of the two binary systems forming initially and one of them merging to one star.
“Such stellar merger is generally predicted to result in a highly-magnetized slowly-rotating blue star on the main sequence,” they said.
“This formation scenario predicts that highly inclined triple- and quadruple-star systems will experience stellar mergers and reduce to binary- and triple-star systems, respectively.”
The team’s paper was published in the Monthly Notices of the Royal Astronomical Society: Letters.
_____
Alejandro Vigna-Gómez et al. Mergers prompted by dynamics in compact, multiple-star systems: a stellar-reduction case for the massive triple TIC 470710327. MNRASL, published online June 29, 2022; doi: 10.1093/mnrasl/slac067
