Mystery of Orion's erupting 'FUor' stars solved after 100 years

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In the vast expanse of the Orion constellation, a peculiar group of stars has captured the attention of astronomers for decades. These stars, known as FUor stars, exhibit sudden and dramatic increases in brightness, defying the typical behavior of young stellar objects.

Now, thanks to the powerful observations made by the Atacama Large Millimeter/submillimeter Array (ALMA), the secrets behind these enigmatic outbursts are finally being unraveled.

Puzzling case of FUor stars

Antonio Hales, deputy manager of the North American ALMA Regional Center and scientist with the National Radio Astronomy Observatory (NRAO), led a research team that delved into the mysteries of FU Orionis, a double star system that first caught astronomers' attention in 1936.

Published in The Astrophysical Journal, their findings shed light on the mechanisms fueling these stellar eruptions.

"FU Ori has been devouring material for almost 100 years to keep its eruption going. We have finally found an answer to how these young outbursting stars replenish their mass," explains Hales. "For the first time we have direct observational evidence of the material fueling the eruptions."

FUor stars and their fuel Source

Using ALMA's advanced capabilities, the researchers detected a long, thin stream of carbon monoxide falling onto FU Orionis.

However, this gas stream alone seemed insufficient to sustain the ongoing outburst. Instead, the team hypothesized that this accretion streamer is a remnant of a much larger feature that previously fell into the young stellar system.

"It is possible that the interaction with a bigger stream of gas in the past caused the system to become unstable and trigger the brightness increase," suggests Hales.

Modeling the accretion streamer

To better understand the dynamics of the mass flow into FU Orionis, the researchers employed novel numerical methods to model the accretion streamer and estimate its properties.

Aashish Gupta, a Ph.D. candidate at the European Southern Observatory (ESO) and co-author of the study, developed these innovative modeling techniques.

"We compared the shape and speed of the observed structure to that expected from a trail of infalling gas, and the numbers made sense," says Gupta.

ALMA's comprehensive view

The observations made by ALMA showcased the instrument's remarkable ability to explore a wide range of angular scales, providing a comprehensive view of star and planet formation dynamics.

Sebastián Pérez of Universidad de Santiago de Chile (USACH) and co-author of the research, highlights the significance of this capability.

"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, spanning from large molecular clouds in which hundreds of stars are born, down to the more familiar scales of solar systems," adds Pérez.

Implications for star and planet formation

Beyond solving the mystery of FU Orionis, these findings have broader implications for our understanding of star and planet formation.

The researchers believe that all stars undergo outburst events, which play a crucial role in shaping the chemical composition of accretion disks and the planets that eventually form around them.

"By understanding how these peculiar FUor stars are made, we're confirming what we know about how different stars and planets form. We believe that all stars undergo outburst events. These outbursts are important because they affect the chemical composition of the accretion disks around nascent stars and the planets they eventually form," emphasizes Hales.

The study of FU Orionis has been a long-term endeavor for the research team, dating back to ALMA's first observations in 2012. Hales expresses his excitement about finally having answers to the puzzling behavior of these outbursting stars.

"We have been studying FU Orionis since ALMA's first observations in 2012," adds Hales. "It's fascinating to finally have answers."

Orion constellation, FU Orionis and future observations

The observations of FU Orionis in the Orion constellation by ALMA have solved a long-standing mystery in astronomy and paved the way for a better understanding of star and planet formation.

By unraveling the secrets behind the enigmatic behavior of FUor stars, researchers have confirmed the crucial role that outburst events play in shaping the chemical composition of accretion disks and the planets that form around them.

As astronomers continue to explore the vast reaches of the universe, the insights gained from studying these peculiar stars will undoubtedly contribute to a more comprehensive understanding of the cosmic processes that govern the birth and evolution of stellar systems.

The full study was published in The Astrophysical Journal.

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