Discovery of a ‘triple star’ could revolutionize understanding of stellar evolution

Artist’s impression composed of a star surrounded by a disk (Be “vampire” star; foreground) and its companion star stripped of its exterior parts (background). Credit: Photo credit: ESO/L. Calçada

A groundbreaking new discovery by scientists at the University of Leeds could transform the way astronomers understand some of the largest and most common stars in the universe. The article, “Gaia discovers the difference in the binarity of B and Be stars on a small scale: evidence of a mass transfer causing the Be phenomenon”, is published in the journal Monthly Notices of the Royal Astronomical Society.

Research by doctorate. Student Jonathan Dodd and Professor René Oudmaijer, from the university’s School of Physics and Astronomy, point to intriguing new evidence that massive Be stars – until now mainly thought to exist in the form of double stars – could actually be “triples”.

This remarkable discovery could revolutionize our understanding of the objects – a subset of B stars – which are considered an important “test bed” for developing theories about how stars evolve more generally.

These Be stars are surrounded by a characteristic disk of gas, similar to the rings of Saturn in our own solar system. And although Be stars have been known for about 150 years – after being first identified by the famous Italian astronomer Angelo Secchi in 1866 – until now, no one knew how they formed.

Until now, the consensus among astronomers has stated that the disks are formed by the rapid rotation of Be stars, and that this itself may be caused by the stars’ interaction with another star in a binary system.

Triple systems

Mr Dodd, corresponding author of the research, said: ‘The best reference point for this is if you’ve watched Star Wars, there are planets where they have two suns.

New research using data from ESO’s Very Large Telescope and Very Large Telescope Interferometer has revealed that HR 6819, previously thought to be a triple system with a black hole, is actually a two-star system without a hole black. The scientists, a team from KU Leuven-ESO, believe they observed this binary system shortly after one of the stars sucked its companion’s atmosphere, a phenomenon often called “stellar vampirism.” This artist’s animation shows what the system could look like; it is composed of an oblate star surrounded by a disk (a Be “vampire” star; foreground) and a type B star that has been stripped of its atmosphere (background). Credit: ESO/L. Calçada.

But now, by analyzing data from the European Space Agency’s Gaia satellite, scientists say they have found evidence that these stars actually exist in triple systems, with three interacting bodies instead of just two.

Mr Dodd added: “We looked at how stars move across the night sky, over longer periods of time, like 10 years, and shorter periods, of around six months. If a star is moving in a straight line, we know there is only one star, but if there are multiple stars, we will see a slight wobble or, in the best case, a spiral.

“We applied this to the two groups of stars that we observe – B stars and Be stars – and what we found, puzzlingly, is that at first glance it appears that Be stars have a lower companion rate than B stars. This is interesting because we would expect them to have a higher rate.

However, lead researcher Professor Oudmaijer said: “The fact that we don’t see them may be because they are now too faint to detect. »

Mass transfer

The researchers then looked at a different data set, looking for more distant companion stars, and found that at these larger separations, the rate of companion stars is very similar between B and Be stars.

They were able to deduce that in many cases a third star comes into play, forcing the companion to move closer to the Be star, close enough that mass can be transferred from one to the other and form the characteristic disk of the star Be. This could also explain why we no longer see these companions; they became too small and faint to detect after the “vampire” star Be sucked up much of their mass.

"Triple star" discovery could revolutionize understanding of stellar evolution

Artist’s impression of a vampire star (left) stealing material from its victim: New research using data from ESO’s Very Large Telescope has revealed that the hottest and brightest stars, known as the name O stars, are often found in close pairs. Many of these binaries will at some point transfer mass from one star to another, a kind of stellar vampirism depicted in this artist’s rendering. Credit: Photo credit: ESO/M. Kornmesser/SE from Mink

This discovery could have huge impacts on other areas of astronomy, including our understanding of black holes, neutron stars and gravitational wave sources.

Professor Oudmaijer said: “There is currently a revolution in physics around gravitational waves. We’ve been observing these gravitational waves for only a few years now, and it was discovered that they were caused by merging black holes.

“We know that these enigmatic objects – black holes and neutron stars – exist, but we don’t know much about which stars might become them. Our findings provide a clue to understanding these sources of gravitational waves.”

He added: “Over the last decade, astronomers have discovered that binarity is an incredibly important element in stellar evolution. We are now moving more towards the idea that it is even more complex than that and that triple stars must be taken into account.”

“Indeed,” Oudmaijer said, “triples have become the new binaries.”

More information:
Jonathan M Dodd et al, Gaia discovers a difference in the binarity of B and Be stars on a small scale: evidence of mass transfer causing the Be phenomenon, Monthly Notices of the Royal Astronomical Society (2023). DOI: 10.1093/mnras/stad3105. On arXiv:

Provided by the University of Leeds

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