Some stars are stuck in bad binary relationships. A massive primary star feeds on its smaller companion, sucking gas from the companion and adding it to its own mass while diminishing its unfortunate partner. These vampire stars are called Be stars, and up until now, astronomers thought they existed in binary relationships.
But new research shows that these stars are only able to feed on their diminutive neighbour because of a third star present in the system.
Be stars are a sub-type of B stars. B is the stars’ spectral type, so both B and Be stars share the same type. Both types are luminous and blue, but while B stars can be from 2-16 times as massive as the Sun, Be stars aren’t as massive. Be stars also rotate more rapidly than other stars, and have accretion rings. About 20% of B stars are Be stars.
Be stars are important in our quest to understand how stars form and evolve. Astronomers have known about Be stars for a long time, but their formation mechanism has been uncertain. Until now.
New research in the Monthly Notices of the Royal Astronomical Society presents evidence that goes a long way to explaining how Be stars form. Its title is “Gaia uncovers difference in B and Be star binarity at small scales: evidence for mass transfer causing the Be phenomenon.” The lead author is Jonathan Dodd, a Ph.D. student at the University of Leeds in the UK.
In general, astrophysicists understand how stars form. A molecular cloud collapses locally into a protostar, which gradually grows more massive over time until fusion is triggered. But there are many variations on that theme, and there are lots of different types of stars in different situations.
Astronomers know that vampire Be stars form accretion rings of superheated gas, and up until now, they thought they had the arrangement figured out. The current understanding is that the Be star’s proximity to the donor star allows the Be star to grow by drawing gas away from the donor into its accretion disk and then into itself. This also increases the Be star’s rotation. Astronomers have found many examples of stripped companion stars around Be stars, adding to the evidence.
But the new research shows that a third star is involved. This enabler star only came into view thanks to the ESA’s Gaia mission. Gaia’s mission is to observe over one billion stars and measure their positions and velocities accurately. “Here, we exploit the exquisite astrometric precision of Gaia to carry out the largest to-date comparative study into the binarity of matched samples of nearby B and Be stars from the Bright Star Catalogue,” the authors write in their paper.
“We observed the way the stars move across the night sky, over longer periods like 10 years, and shorter periods of around six months,” Dodd said. “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.”
“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,” Dodd said. “This is interesting because we’d expect them to have a higher rate.”
If Be stars grow larger because they draw material away from a donor star, then, of course, Be stars should have more binary partners than B stars. Maybe they’re there, but they’re just difficult to detect.
“The fact that we do not see them might be because they are now too faint to be detected,” said Professor René Oudmaijer, the study’s co-author.
The researchers dug deeper into data, looking for the binary companions of Be stars that might be further away. They found that at larger separations, the rate of companion stars is much more similar between B and Be stars. From this, they inferred that a third star is involved, and it’s actually the one they’re seeing at a larger separation.
They think that interactions with the third star force the donor star to come closer to the vampire star. As the donor draws closer to the vampire star, the vampire star siphons off material into its accretion disk. As a result, the donor star is so small and faint that it becomes difficult to observe.
The companion stars that the team discovered when they widened their search are too distant from the vampire star for the vampire star to siphon off mass. But astronomers know that a third star can drive a binary pair closer together and also ‘harden’ the link between the inner pair. “It is well known that higher-order multiplicity can result in the hardening of an inner binary,” the authors explain in their research. “Indeed, a third body increases the occurrences of migration and eventual binary interactions significantly.”
There are a couple of ways the scenario can play out. When a system eventually forms a stripped binary, the mass transfer can take place between the inner pair, and the outer third star can become unbound. Or, the inner binary can actually merge; the outer third star migrates closer to the primary star, and the migrated star can become the new donor.
Either way, binary interactions are responsible for Be stars. “Our results therefore imply that close binary interactions are responsible for the formation of Be stars,” the authors write. “Moreover, we suggest that triplicity must play a vital role in triggering this migration and thus in the formation of Be stars as a whole.”
The discovery not only sheds light on how Be stars come to be. It also sheds light on gravitational waves. Gravitational waves are created when two massive objects merge, like a pair of black holes, a pair of neutron stars, or one of each.
If stripped companions are present near Be stars, and a third star is needed for the scenario to play out, then can this paint a clearer picture of the progenitors of some of the dense objects that lead to gravitational waves?
“There’s a revolution going on in physics at the moment around gravitational waves,” Professor Oudmaijer said. “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,” Oudmaijer added.
“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,” Oudmaijer said.?
“Indeed,” he added, “triples have become the new binaries.”