Be-type stars make up almost 20% of the B star population, and are rapidly rotating stars surrounded by a disk. Although these stars have been known for about 150 years — having first been identified by the renowned Italian astronomer Angelo Secchi in 1866 — until now, no one has known how they were formed.
An artist’s impression of HR 6819, a close binary consisting of a stripped B-type star (background) and a rapidly rotating Be star (foreground). Image credit: ESO / L. Calçada.
“The best point of reference for that is if you’ve watched Star Wars, there are planets where they have two Suns,” said University of Leeds Ph.D. student Jonathan Dodd.
“But now, by analyzing data from ESA’s Gaia satellite, we found evidence these stars actually exist in triple systems — with three bodies interacting instead of just two.”
“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.”
“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.”
“The fact that we do not see them might be because they are now too faint to be detected,” said University of Leeds Professor René Oudmaijer.
The astronomers then looked at a different set of data, looking for companion stars that are further away, and found that at these larger separations the rate of companion stars is very similar between the B and Be stars.
From this, they were able to infer that in many cases a third star is coming into play, forcing the companion closer to the Be star — close enough that mass can be transferred from one to the other and form the characteristic Be star disk.
This could also explain why we do not see these companions anymore; they have become too small and faint to be detected after the ‘vampire’ Be star has sucked in so much of their mass.
The discovery could have huge impacts on other areas of astronomy — including our understanding of black holes, neutron stars and gravitational wave sources.
“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.”
“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.”
The findings were published in the Monthly Notices of the Royal Astronomical Society.
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Jonathan M. Dodd et al. 2024. Gaia uncovers difference in B and Be star binarity at small scales: evidence for mass transfer causing the Be phenomenon. MNRAS 527 (2): 3076-3086; doi: 10.1093/mnras/stad3105
