An unusual group of stars in the Orion constellation has revealed their secrets. FU Orionis, a double star system, first caught the attention of astronomers in 1936, when the central star suddenly became 1,000 times brighter than usual. This behavior, expected of dying stars, has never been seen in a young star like FU Orionis. Now researchers have solved the mystery
An artist's impression of the large-scale landscape of FU~Ori. The image shows the currents created by the interaction between strong stellar winds triggered by the outburst and the remnant mantle from which the star formed. The stellar wind drives the shock wave into the mantle and the CO gas carried away by the shock is the new discovery of the ALMA observatory. Credit: NSF/NRAO/SDagnello
ALMA observations of FU Orionis reveal how gravitational accretion from a past gas stream causes sudden brightness in young stars, shedding light on the processes of star and planet formation.
An unusual group of stars in the Orion constellation has revealed their secrets. FU Orionis, a double star system, first caught the attention of astronomers in 1936, when the central star suddenly became 1,000 times brighter than usual. This behavior, expected of dying stars, has never been seen in a young star like FU Orionis.
The strange phenomenon inspired a new classification of stars with the same name (FUor stars). FUor stars flare up suddenly, bursting with brightness, before dimming again for many years afterwards.
It is now understood that this brightening is due to the stars absorbing energy from their surroundings through gravitational accretion, the main force that shapes stars and planets. However, how and why this happens remains a mystery – until now, thanks to astronomers using the Atacama Large Millimeter/Submillimeter Array (ALMA).
Groundbreaking observations with ALMA
“FU Ori has been pumping material into it for almost 100 years to keep it erupting. We finally found an answer to the question of how these young exploding stars renew their mass," explains Antonio Hales, deputy director of the Alma Regional Center in North America, scientist at the National Radio Astronomy Observatory, and lead author of this study, published on April 29 in the Astrophysical Journal. "For the first time, we have direct observational evidence of the material that fuels the eruptions."
A close look at the FU Ori binary system and the recently discovered accretion trail. This artist's impression shows the newly discovered trail continuously feeding mass from the envelope into the binary system. Credit: NSF/NRAO/S. Dagnello
ALMA observations have revealed a long, thin stream of carbon monoxide extending into FU Orionis. It appears that this gas did not have enough fuel to sustain the current eruption. Instead, this accretion trail is likely the remnants of an earlier, much larger interstellar component that was engulfed within this young star system.
"It is possible that the interaction with a larger stream of gas in the past made the system unstable and caused an increase in brightness," Hales explains.
Advances in understanding star formation
Astronomers used several configurations of ALMA antennas to capture the different types of emission coming from FU Orionis, and detect the mass flow into the star system. They also incorporated innovative numerical methods to model the mass flow as an accretion current and evaluate its properties.
"We compared the shape and speed of the observed structure to that expected from a cloud of infalling gas, and the numbers made sense," says Ashish Gupta, a doctoral student at the European Southern Observatory (ESO), and one of the authors of the paper, who developed the methods used to model the accretion current.
A close look at the FU Ori binary system and the newly discovered accretion trail. This artist's impression shows the newly discovered trail continuously feeding mass from the envelope into the binary system. Credit: NSF/NRAO/S. Dagnello
"Alma gives us a comprehensive look at the dynamics of star and planet formation, from large molecular clouds where hundreds of stars are born, to the more familiar scales of solar systems," adds Sebastian Perez of the University of Santiago de Chile (USACH), Director of the Millennium Core on Young Planets and Their Moons (YEMS) in Chile, and co-author of this study.
These observations also revealed a flow of carbon monoxide moving more slowly than FU Orionis. This gas is not related to the recent eruption. In fact, it resembles currents observed around other protostellar objects.
Hales adds: "By understanding how these strange FUor stars form, we are confirming what we know about how different stars and planets form. We believe that all stars undergo outburst events. These eruptions are important because they affect the chemical composition of the 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 get answers."
