Astronomers believe that when high-mass stars form, it's not so much a smooth collapse of gas but more a sequence of short, infrequent bursts of material, leading to a sudden increase in mass and a release of intense heat. This phenomenon, called an accretion burst, was first observed in 2016 and now an international team of researchers has spotted the third one to ever be observed.
As reported in Nature Astronomy, the observations focus on a massive protostar called G358-MM1, which is located 22,000 light-years from Earth. Last January, astronomers at Ibaraki University in Japan noticed new activity around the star, a possible indication of an accretion burst. An international collaboration of astronomers called the Maser Monitoring Organisation (M2O) spent some time using several radio telescopes in New Zealand, Australia, and South Africa. By using them together they were able to spot subtle changes in the accretion burst.
"The M2O observations are the first to witness the immediate aftermath of an accretion burst in a high-mass protostar in such detail, which provide evidence in support of the 'episodic accretion' theory of high-mass star formation," lead author Ross Burns, from the National Astronomical Observatory of Japan, said in a statement. "Our team greatly benefits from close communication between a diverse, global community of observers, astrophysicists and theorists in planning, executing and interpreting transient maser events."
This work is now being followed up with a series of deeper investigations. These will be crucial to solving an important mystery related to massive stars. Our models suggest that due to the intense radiation emitted, a star should not grow much bigger than eight times the mass of our own Sun. But high-mass stars don’t seem to care about our models, and we know that stars tens of times the mass of the Sun exist out there.
The tension between model and reality is a long-standing problem. Accretion bursts could be the crucial mechanism that solves it. The idea is that these heavy protostars are surrounded by a disk of material, and once in hundreds if not thousands of years, some of this material will fall onto the star, increasing its mass.
The discovery of three bursts suggests that this is the case but more observations are necessary. These won’t be easy though. By their very nature, these events happen occasionally and massive protostars are shrouded in thick gas clouds, making them difficult to study with optical telescopes.