SIX POTENTIAL EXPLANATIONS
What could have caused the star to dim?
An unknown dark object passed by
A simple explanation is that an unknown dark object drifted in front of the giant star, and this just happened to be caught in the data. The astronomers have ruled this theory out, however, because computer simulations show there would need to be “an implausibly large number of dark bodies floating around the galaxy for this scenario to be likely,” says Smith.
The star itself was surrounded by dust
Maybe a circumstellar disc of dust and debris obscured VVV-WIT-08, causing it to dim. But this wasn’t seriously considered, says Smith, because it would require huge changes in disc thickness to cause such a large variation in brightness. It would need to be thin outside of the dimming event then as thick as the giant star is large during it. “I wouldn’t expect such a disc to be stable long term,” he says.
The star was obscured by dust from elsewhere
It’s generally felt that the obscuring material is dust. This might be thick enough to let no light through, or it might allow a tiny amount of light through if the grains are large. But grains smaller than a few micrometres would block more visible light than infrared, which doesn’t match the observations, says Smith
A black hole is orbiting the star
Perhaps an accretion disc around a black hole is to blame for the dimming of VVVWIT-08. But the researchers’ paper talks of a black hole needing to be in the low tens of solar masses – relatively small in black hole terms. Yet X-rays would be emitted, and none were present. Astronomers have never observed a dark, dense debris ring around an orbiting black hole either, so the jury remains out on this one.
It’s obscured by matter originating from the star
As a companion object orbited the star, there could have been a mass transfer of matter to it which created a disc. Their current separation is too great to facilitate this – but it might not have been in the past. A transfer of matter such as this would conveniently explain the potential smaller size of the giant star.
It’s obscured by matter originating from a companion
There is a possibility that the material blocking the star’s light came from an object gravitationally bound to VVV-WIT-08. The material could have come from the companion, ejected when it underwent a supernova explosion. Or it might have been ejected during the less dramatic later stages of its evolution.