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\u2019s 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. <\/p>\n
<\/p>\n The star in question is called Orionis North, and it\u2019s 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. <\/p>\n Astronomers working with ALMA (Atacama Large Millimetre-submillimetre Array) have discovered the reason behind Fu Ori\u2019s variability. They\u2019ve published their research in the Astrophysical Journal. It\u2019s titled \u201cDiscovery of an Accretion Streamer and a Slow Wide-angle Outflow around FU Orionis,\u201d and the lead author is Antonio Hales, deputy manager of the North American ALMA Regional Center and scientist with the NRAO.<\/p>\n Here\u2019s 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\u2019t understand was why and how this happened.<\/p>\n \u201cFU 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,\u201d explained lead author Hales. \u201cFor the first time we have direct observational evidence of the material fueling the eruptions.\u201d<\/p>\n ALMA is the world\u2019s largest radio telescope. It\u2019s an interferometer with 66 separate antennae, which can be moved across the ground to give the observatory a \u2018zoom-in\u2019 effect. This powerful observatory has driven a lot of astronomical science. <\/p>\n In this research, ALMA identified a long streamer of carbon monoxide that appears to be falling into FU Ori. The researchers don\u2019t think this streamer has enough material to sustain the star\u2019s current outburst. But it could be the remnant from a past episode. \u201cIt 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,\u201d explained Hales.<\/p>\n The current outburst creates strong stellar winds that interact with a leftover envelope of material from the star\u2019s formation. The wind shocks the envelope, sweeping up carbon monoxide with it. The CO is what ALMA detected. <\/p>\n ALMA\u2019s 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. \u201cWe compared the shape and speed of the observed structure to that expected from a trail of infalling gas, and the numbers made sense,\u201d 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. <\/p>\n The researchers measured the amount of material flowing into FU Ori through the streamer. About 0.07 Jupiter Masses per Myr?1<\/sup> flow into the young star. Jupiter is about 318 times more massive than Earth. This means that FU Ori\u2019s infall streamer rate is lower than infall around other Class 0 protostars. \u201cThis 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,\u201d the authors point out. <\/p>\n The infall streamer and its effect on the star are complex. Not enough material comes in via the streamer to trigger the outbursts. \u201cThe streamer needs to be more massive to sustain FU Ori\u2019s 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,\u201d the authors explain. <\/p>\n Instead, the infalling material causes disk instability, which in turn delivers enough material to FU Ori to trigger outbursts. \u201cAnisotropic 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,\u201d Hales and his co-authors write. They can\u2019t say for sure if this is what\u2019s happening. That would require more modelling, which is outside the scope of this work. <\/p>\n 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\u2019t related to the brightening. \u201cThe ALMA observations reveal the presence of large-scale, wide-angle bipolar outflows for the first time around the class prototype FU Ori,\u201d the researchers write in their paper. <\/p>\n Curiously, astronomers have detected these outflows from other FUor stars but never at FU Ori itself. It\u2019s coming from Fu Ori North, the star that experiences the powerful brightening. <\/p>\n \u201cPrior 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,\u201d the researchers write in their paper. \u201cThese 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.\u201d<\/p>\n Understanding young stars is critical because their behaviour governs planet formation. FU Ori\u2019s brightening could have a defining effect on the planets that form around the star. <\/p>\n![\"Artist's](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2024\/05\/Hales_Illustration_cutout-1024x580-1.jpg\")
![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2024\/05\/FU-Ori-infalling-streamer-model.jpg\")