“Triple Star” Discovery Could Revolutionise Understanding of Stellar Evolution
Although star formation may be seen as trivial for stars like our sun, some types are more elusive than others. Massive Be type stars are one such example often found in binary systems, where two stars obit each other. “The best point of reference for that is if you’ve watched Star Wars, there are planets where they have two Suns” explains Johnathan Dodd, PhD student and collaborator on the research. However, not much is known about their formation.
A recent discovery was made by Professor René Oudmaijer, PhD student Johnathan Dodd along with PhD student Isaac Radley from the University of Leeds’ School of Physics and Astronomy and two former Leeds academics Dr Miguel Vioque of the ALMA Observatory in Chile and Dr Abigail Frost at the European Southern Observatory in Chile. It suggests that these stars may also exist in triplet systems (where there are three stars orbiting each other), as well as double. This may help to reveal the long-awaited mystery of their formation since their identification via their emission lines in 1866 by Angelo Secchi.
The “triple star” discovery by Professor Oudmaijer and his team was made by using European Space Agency’s Gaia satellite and could cause ripples of excitement in the astrophysics community. It may also aid our understanding of neutron stars, gravitational waves, and black holes.
Principal Investigator Professor Oudmaijer said “there’s a revolution going on in physics at the moment around gravitational waves. We have only been observing these gravitational waves for a few years now, and these have been found to be due to merging black holes.”
“We know that these enigmatic objects – black holes and neutron stars – exist, but we don’t know much about the stars that would become them. Our findings provide a clue to understanding these gravitational wave sources.”
He added that “over the last decade or so, astronomers have found that binarity is an incredibly important element in stellar evolution. We are now moving more towards the idea it is even more complex than that and that triple stars need to be considered.”
“Indeed,” Oudmaijer said, “triples have become the new binaries”.
New research using data from ESO’s Very Large Telescope and Very Large Telescope Interferometer has revealed that HR 6819, previously believed to be a triple system with a black hole, is in fact a system of two stars with no black hole. The scientists, a KU Leuven-ESO team, believe they have observed this binary system in a brief moment after one of the stars sucked the atmosphere off its companion, a phenomenon often referred to as “stellar vampirism”. This animation shows what the system might look like; it’s composed of an oblate star with a disc around it (a Be “vampire” star; foreground) and B-type star that has been stripped of its atmosphere (background).
Their research involved looking for further away secondary stars within various data sets to build a picture that at larger separations the number of companions stars is comparable between Herbig Be and B type stars.
“We observed the way the stars move across the night sky, over longer periods like 10 years, and shorter periods of around six months. If a star moves in a straight line, we know there’s just one star, but if there is more than one, we will see a slight wobble or, in the best case, a spiral.” Mr Dodd explained.
The two types of stars they were most interested in are B and Be; the former displays hydrogen emission lines and the latter are main sequence non-supergiant stars that have Balmer series emission lines.
“We applied this across the two groups of stars that we are looking at – the B stars and the Be stars – and what we found, confusingly, is that at first it looks like the Be stars have a lower rate of companions than the B stars. This is interesting because we’d expect them to have a higher rate” Mr Dodd added.
In a large proportion of the sample analysed had the inference that a third body was influencing the system, resulting in the Be star having a reduced distance from its companion. This causes mass transfer from the two stars as the Be star ‘sucks’ matter from the other. It’s been dubbed a “vampire” Be star and this behaviour forms the characteristic disc around the Be star.
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, which are known as O stars, are often found in close pairs. Many of such binaries will at some point transfer mass from one star to another, a kind of stellar vampirism depicted in this artist’s impression.
A reason as to why the companion is hard to spot could be due to them being obscured by the disc and reduced so much by the Be star making them very small and faint.
Professor Oudmaijer said “the fact that we do not see them might be because they are now too faint to be detected.”
This breakthrough has opened up another area of exciting research.
Their paper entitled “Gaia uncovers difference in B and Be star binarity at small scales: evidence for mass transfer causing the Be phenomenon” was published on the 21st November and can be found here.
