The discovery of ALMA: the secret of the stars FU Orionis
The observations carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) have allowed to reveal the mechanism underlying the sudden increase in brightness of young stars, such as those of the FU Orionis system, providing new information on the processes of star and planetary formation.
An unusual group of stars in the constellation Orion has revealed its secrets. FU Orionis, a binary star system, caught the attention of astronomers in 1936 when the central star suddenly became 1,000 times brighter than usual. This behavior, expected from dying stars, had never been observed in a star as young as FU Orionis.
The phenomenon has inspired a new classification of stars with the same name (FUor stars). FUor stars suddenly light up, erupting in brightness, before dimming again many years later.
It is now understood that this increase in brightness is due to stars absorbing energy from their surrounding environment via gravitational accretion, the main force that shapes stars and planets. However, the way Why this happens has remained a mystery until now, thanks to astronomers using ALMA.
Breakthrough observations with ALMA
“FU Ori has been devouring material for almost 100 years to maintain its eruption. We have finally found an answer to how these young erupting stars replenish their mass,” explains Antonio Hales, deputy director of the North American ALMA Regional Center, scientist at the National Radio Astronomy Observatory and lead author of this research , published on 29 April in the magazine Astrophysical Journal. “For the first time, we have direct observational evidence of the material fueling the eruptions.”
ALMA observations revealed a long, thin stream of carbon monoxide falling on FU Orionis. This gas did not appear to have enough fuel to sustain the current eruption. Instead, this accretion flow is believed to be a remnant of an earlier, much larger feature that fell into this young star system.
“It’s possible that interaction with a larger flow of gas in the past caused the system to become unstable and triggered the brightening,” explains Hales.
Advances in understanding star formation
Astronomers used different configurations of ALMA antennas to capture different types of emission from FU Orionis and detect mass flow in the stellar system. They also combined innovative numerical methods to model the mass flow as an accretion flow and estimate its properties.
“We compared the shape and velocity of the observed structure with that expected from a trail of infalling gas, and the numbers made sense,” says Aashish Gupta, a doctoral candidate at the European Southern Observatory (ESO) and co-author of this work, which developed the methods used to model the accretion flow.
“The range of angular scales we are able to explore with a single instrument is truly remarkable. ALMA gives us a comprehensive view of the dynamics of star and planet formation, ranging from large molecular clouds in which hundreds of stars are born to the more familiar scales of solar systems, he adds Sebastián Pérez of the Universidad de Santiago de Chile (USACH), director of the Millennium Nucleus on Young Exoplanets and their Moons (YEMS) in Chile and co-author of this research.
These observations also revealed a slow-velocity carbon monoxide flow from FU Orionis. This gas is not associated with the latest eruption. Instead, it is similar to flows observed around other protostellar objects.
Concludes Hales, “By understanding how these peculiar FUor stars form, we are confirming what we know about the formation of different stars and planets. We believe that all stars undergo eruption events. These eruptions are important because they affect the chemical composition of accretion disks around nascent stars and the planets they eventually form.”
“We have been studying FU Orionis since the first ALMA observations in 2012,” adds Hales. It’s fascinating to finally have some answers.”
