Astronomers have long suspected some red giant stars are losing a lot of mass to their companions, but confirming this has been a challenge. Now, however, a study of red giants imaged by the Kepler Space Telescope has revealed two distinct populations of stars that show tell-tale signs of having lost a lot of their material. In the process, the researchers have identified a tell-tale feature of such stars, which could make them easier to find in future. They've also demonstrated, once again, Kepler's flexibility, being used for research far beyond its original intent.
When stars age they cool and puff up, becoming red giants. Although each square meter of their surface is putting out less light than in more youthful times, these giants are so, well, gigantic, they represent the brightest point in their lifespan, unless it becomes a supernova. That makes red giants highly visible over great distances. Although accounting for a tiny proportion of stars, that visibility means we can see many of them, as well as providing exceptional opportunities to study stellar evolution and behavior.
Being so diffuse, however, it is to be expected that red giants in close binary systems will lose mass to denser neighbors. Astronomers have been keen to study the process but struggled to identify cases where it has occurred since stars evolve far too slowly for us to witness it in real-time. Now, however, two populations of red giants that have undergone dramatic mass loss have been identified in Nature Astronomy.
In order to find these denuded stars, University of Sydney PhD student Yaguang Li and co-authors searched through 7,000 red giants that appeared in Kepler's field of view while the telescope was looking for signs of planets partially blocking stars.
“It’s like finding Waldo,” Li said in a statement. “We were extremely lucky to find about 40 slimmer red giants, hidden in a sea of normal ones.”
One of the populations – dubbed very low-mass stars – could be identified by having masses 0.5-0.7 times that of the Sun. Smaller stars evolve far more slowly than more massive ones. Below a certain mass, a star should have taken longer than the age of the universe to reach the red giant phase. Clearly, this is impossible, so the star must once have had the extra material that would drive it to evolve through its early life faster.
During the red giant phase, stars throw off some of their mass on their own in an enhanced version of the solar wind. However, astronomers are confident the loss never exceeds 0.1 solar masses for stars of the size studied here, unless there is outside assistance. Li and co-authors identified 32 stars that are simply too low mass to have reached their current state on their own – only a thieving partner could explain the situation, whether we can see one or not.
Members of the second category – underluminous stars – were even harder to spot. They're not as bright as expected. “They’ve slimmed down somewhat and because they’re smaller, they’re also fainter, hence ‘underluminous’ compared to normal red giants,” co-author Dr Simon Murphy of the University of Southern Queensland said. To identify them we need to establish expectations, which the authors did through astroseismology – the study of vibrations produced by sound waves within stars – which can reveal information about the helium-burning core.
Stars that start life with between 1.8 and 3.6 stellar masses have distinctive cores, and in seven cases Li noticed stars with cores like this, but now possessing masses below the threshold.
The good news for those keen to explore red giant evolution is that one of the underluminous stars is very high in lithium for a red giant. The authors think this could offer a quicker path to future identification of stars whose partner stole their outer layers.