Located about 1,360 light-years from the Solar System, FU Orionis, also known as FU Ori, is a variable star that mysteriously became 1,000 times brighter 88 years ago. Astrophysicists have been trying to find out why for a long time, especially for over a decade, using the Atacama Large Millimeter/submillimeter Array (Alma) network of radio telescopes. A once proposed hypothesis has gained more weight.
A press release from the National Radio Astronomy Observatory discusses work done with the Atacama Large Millimeter/submillimeter Array (Alma) of the ESO and published in an article in The Astrophysical Journal. It discusses observations confirming a hypothesis put forward to solve an 80-year-old mystery concerning a star in the Milky Way capable of producing solar eruptions equivalent but a thousand billion times stronger than those of the Sun.
It all began in 1936 when the star FU Orionis located in the Orion constellation, inside the dark Barnard 35 nebula (a cold molecular and dusty cloud), suddenly increased its brightness hundreds of times, reducing its apparent magnitude from 16.5 to 9.6 in a little under 200 days. Initially, astronomers thought they were witnessing a kind of supernova, but a more in-depth study of the phenomenon, particularly with spectral signatures, showed otherwise.
FU Ori a Double Proto-Star?
For decades, FU Orionis remained a unique case, but in 1970, a similar star – V1057 Cygni – was discovered. Now, around ten stars have exhibited the same behavior and are grouped in a new class of
The stars known as FU Orionis variables. These FUors, as they are called, are now well known to amateur astronomers specializing in variable stars and are members of the famous American Association of Variable Star Observers (AAVSO).
It is now known that FU Orionis is actually a binary system of young stars, specifically with the star responsible for its unique behavior being a T Tauri protostar not yet on the main sequence, deriving its energy from ongoing accretion and gravitational contraction processes rather than hydrogen fusion reactions.
The current model to understand the sudden brightness changes of FUors was mainly developed by American astrophysicists Lee Hartmann and Scott Jay Kenyon, associating the FU Orionis eruption with a sudden mass transfer from an accretion disk to a low-mass young T Tauri star.
“A stream of accretion betrayed by carbon monoxide emissions”
In a statement from the National Radio Astronomy Observatory, Antonio Hales, deputy director of the North American ALMA Regional Center and lead author of the article in the Astrophysical Journal, explains today that “FU Ori has been devouring matter for nearly 100 years to sustain its eruption. We finallyfound an answer on how these young shining stars rebuild their mass. For the first time, we have direct observational evidence of the matter fueling the eruptions.
Alma’s observations have indeed revealed a long and thin stream of carbon monoxide CO falling onto FU Orionis. However, this stream of matter does not seem to contain enough to explain the brightness of FU Ori, but, according to Hales, “it is possible that interaction with a larger gas flux in the past made the system unstable and triggered an increase in brightness.
By understanding how these particular FUor stars are made, we confirm what we know about the formation of different stars and planets. We believe that all stars undergo explosions. These explosions 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. It’s fascinating to finally have answers”.