Astronomers solve the mystery of the dramatic 1936 explosion of FU Orionis

Artist's impression of the large-scale view of FU~Ori. The image shows outflows resulting from the interaction between the powerful stellar wind powered by the explosion and the remaining atmosphere from which the star formed. Stellar winds create a powerful shock inside the atmosphere, and the carbon dioxide gas swept up by the shock is what the new ALMA has revealed. Credit: NSF/NRAO/S. Danilo

Alma FU Orionis observations reveal how gravitational accretion from a past gas stream causes sudden brightenings in young stars, shedding light on star and planet formation processes.

An unusual group of stars in the Orion constellation has revealed its 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 normal. This behavior, expected from dying stars, has never been seen before in a young star like Vo Orionis.

This strange phenomenon has inspired a new classification of stars with the same name (FUor Stars). The stars suddenly glow, exploding in brightness, before dimming again after many years.

It is now understood that this brightness is because stars derive energy from their surroundings via gravitational accretion, the main force that forms stars and planets. However, how and why this happened has remained a mystery – until now, thanks to astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA).

Breakthrough notes with ALMA

FU Ori has been gobbling up material for nearly 100 years to keep its eruption going. “We have finally found the answer to how these young stars replenish their mass,” explains Antonio Hales, deputy director of the ALMA North America Regional Center and a scientist at the National Astronomy Observatory. Al-Radawi, the lead author of this research, which was published on April 29 in the Astrophysical Journal. “For the first time, we have direct observational evidence of the materials fueling the explosions.”

Zoom in on the FU Ori binary system and its newly discovered accumulator. This artist's impression shows the newly discovered streamer continuously feeding mass from the shell to the binary system. Credit: NSF/NRAO/S. Danilo

ALMA observations revealed a long, thin stream of carbon monoxide falling on FU Orionis. It appears that this gas does not contain enough fuel to withstand the current explosion. Instead, this accretion flow is thought to be a remnant of a much larger, earlier feature that fell into this young star system.

“It is possible that interaction with a larger stream of gas in the past may have destabilized the system and caused the brightness to increase,” Hales explains.

Advances in understanding star formation

Astronomers have used several configurations of ALMA antennas to capture different types of emissions coming from FU Orionis, and detect the flow of mass into the star system. They also incorporated new numerical methods to model mass flow as a cumulative flow and estimate its properties.

“We compared the shape and velocity of the observed structure with those expected from a cascade of falling gases, and the numbers made sense,” says Ashish Gupta, Ph.D. candidate at the European Southern Observatory (Eso), and co-author of this work, who developed the methods used to model the cumulative emission device.

Zoom in on the FU Ori binary system and its newly discovered accumulator. This artist's impression shows the newly discovered streamer continuously feeding mass from the shell to the binary system. Credit: NSF/NRAO/S. Danilo

“The range of angular scales we can explore with a single instrument is truly remarkable,” adds Sebastian Pérez of the University of Santiago de Chile (USACH). “ALMA gives us a comprehensive view of the dynamics of star and planet formation, from the large molecular clouds in which hundreds of stars are born, all the way to The most common metrics for solar systems.”, director of the Millennium Nucleus of Young Exoplanet and Their Moons (YEMS) in Chile, and co-author of this research.

These observations also revealed a slow-moving carbon monoxide flow from FU Orionis. This gas is not related to the recent explosion. Instead, it resembles outflows observed around other stellar protobodies.

“By understanding how these strange stars are made, we confirm what we know about how different stars and planets form,” Hales adds. “We believe that all stars undergo explosive events. These explosions are important because they affect the chemical composition of the accretion disks around the emerging stars and the planets they form in.” the end.

“We have been studying FU Orionis since the first ALMA observations in 2012,” Hales adds. It's great that we're finally getting answers.

Reference: “Detection of a slow wide-angle accretion and jet device around FU Orionis” by A. S. Hales, A. Gupta, D. Ruíz-Rodríguez, J. P. Williams, S. Pérez, L. Cieza, C. González-Ruilova, J. E. Pineda, a. Santamaria-Miranda, J. Tobin, B. Weber, Z. Zhou, and A. Zorlu, April 29, 2024, Astrophysical Journal.
doi: 10.3847/1538-4357/ad31a1