A Star Became 1,000 Times Brighter, and Now Astronomers Know Why

Astronomers were surprised in 1937 when a star in a binary pair suddenly brightened by 1,000 times. The pair is called FU Orionis (FU Ori), and it’s in the constellation Orion. The sudden and extreme variability of one of the stars has resisted a complete explanation, and since then, FU Orionis has become the name for other stars that exhibit similar powerful variability.

The star in question is called Orionis North, and it’s the central star of the pair. Astronomers see its brightening behaviour in old stars but not in young stars like FU Ori. The young star is only about 2 million years old.

Astronomers working with ALMA (Atacama Large Millimetre-submillimetre Array) have discovered the reason behind Fu Ori’s variability. They’ve published their research in the Astrophysical Journal. It’s titled “Discovery of an Accretion Streamer and a Slow Wide-angle Outflow around FU Orionis,” and the lead author is Antonio Hales, deputy manager of the North American ALMA Regional Center and scientist with the NRAO.

Here’s what scientists do know about FU Ori (FUor) stars and their variability. They brighten when they attract gas gravitationally into an accretion disk. Too much mass at once can destabilize the disk, and as material falls into the star, it brightens. But what they didn’t understand was why and how this happened.

“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,” explained lead author Hales. “For the first time we have direct observational evidence of the material fueling the eruptions.”

ALMA is the world’s largest radio telescope. It’s an interferometer with 66 separate antennae, which can be moved across the ground to give the observatory a ‘zoom-in’ effect. This powerful observatory has driven a lot of astronomical science.

In this research, ALMA identified a long streamer of carbon monoxide that appears to be falling into FU Ori. The researchers don’t think this streamer has enough material to sustain the star’s current outburst. But it could be the remnant from a past episode. “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,” explained Hales.

This figure from the research shows 12CO and 13CO emissions as detected by ALMA. The colours denote velocity. The CO streamer of infalling gas is labelled. “The elongated feature has a connection neither to the larger-scale molecular outflow nor to the inner disk rotation and is more similar to accretion streamers recently reported around young stellar objects,” the authors explain. Image Credit: Hales et al. 2024.

The current outburst creates strong stellar winds that interact with a leftover envelope of material from the star’s formation. The wind shocks the envelope, sweeping up carbon monoxide with it. The CO is what ALMA detected.

Artist’s impression of the large-scale view of FU~Ori. The image shows the outflows produced by the interaction between strong stellar winds powered by the outburst and the remnant envelope from which the star formed. The stellar wind drives a strong shock into the envelope, and the CO gas swept up by the shock is what the new ALMA revealed. The inset image is an artist’s impression of the streamer of CO feeding mass into FU Ori. Image Credit: NSF/NRAO/S. Dagnello

ALMA’s ability to operate in different configurations and wavelengths played a role in this work. It allowed the team to detect different types of emissions and to detect the mass flowing into FU Ori. They compared the observations to models of mass flow and accretion streamers. “We compared the shape and speed of the observed structure to that expected from a trail of infalling gas, and the numbers made sense,” said Aashish Gupta, a Ph.D. candidate at European Southern Observatory (ESO). Gupta is a co-author of this work, and he developed the methods used to model the accretion streamer.

This image from the research shows the model results (green line) overlain on ALMA data. The streamer modelling closely matches the data. “The fitting results suggest that the morphology and the velocity profile of the observed streamer emission can be well represented as a trail of infalling gas,” the authors write in their published research. Image Credit: Hales et al. 2024.

The researchers measured the amount of material flowing into FU Ori through the streamer. About 0.07 Jupiter Masses per Myr?1 flow into the young star. Jupiter is about 318 times more massive than Earth. This means that FU Ori’s infall streamer rate is lower than infall around other Class 0 protostars. “This would suggest that the observed streamer will require ?100 Myr to replenish disk masses, which is at least an order of magnitude greater than the typical disk lifetimes,” the authors point out.

The infall streamer and its effect on the star are complex. Not enough material comes in via the streamer to trigger the outbursts. “The streamer needs to be more massive to sustain FU Ori’s outburst accretion rates (by several orders of magnitude). The estimated streamer mass infall rate is not even sufficiently massive to sustain quiescent stellar accretion rates,” the authors explain.

Instead, the infalling material causes disk instability, which in turn delivers enough material to FU Ori to trigger outbursts. “Anisotropic infall, cloudlet capture events, the inhomogeneous delivery of material, and the building up of material around dust traps can all lead to the disk instabilities that could trigger accretion outbursts,” Hales and his co-authors write. They can’t say for sure if this is what’s happening. That would require more modelling, which is outside the scope of this work.

ALMA also spotted another streamer of slow-moving CO. This one is coming from the star rather than falling into it. Hales and his colleagues think this streamer is similar to streamers coming from other young protostellar objects and isn’t related to the brightening. “The ALMA observations reveal the presence of large-scale, wide-angle bipolar outflows for the first time around the class prototype FU Ori,” the researchers write in their paper.

Curiously, astronomers have detected these outflows from other FUor stars but never at FU Ori itself. It’s coming from Fu Ori North, the star that experiences the powerful brightening.

“Prior searches for molecular outflows around FUors, mainly using single-dish telescopes, reported outflowing material from many FUors but failed to detect flows emerging from the FUor class prototype,” the researchers write in their paper. “These nondetections instigated the belief that there were no molecular outflows around the FU Ori system. Our discovery ends the mystery by clearly demonstrating the presence of a molecular outflow from FU Ori itself.”

Understanding young stars is critical because their behaviour governs planet formation. FU Ori’s brightening could have a defining effect on the planets that form around the star.

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

For the authors, their research demonstrates how the powerful ALMA observatory makes a unique contribution to astronomical research. “These results demonstrate the value of multiscale interferometric observations to enhance our understanding of the FU Ori outbursting system and provide new insights into the complex interplay of physical mechanisms governing the behaviour of FUor-type and the many other kinds of outbursting stars,” the authors conclude.

Evan Gough

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