ALMA solves 88-year-old astronomical mystery

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have finally solved an 88-year-old astronomical mystery surrounding the erupting star system in the Orion constellation.

In 1936, a young star FU Orionis in the Orion constellation began gobbling material from its surrounding disk and became 1,000 times brighter than usual. FUor stars, a category of stars sharing the same name, flare suddenly, erupting in brightness, before dimming again many years later.

This eruption is caused by stars drawing in energy through gravitational accretion, a process that shapes stars and planets. However, the reasons behind this phenomenon remained unknown - until now.

"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 Antonio Hales, Deputy Manager of the North American ALMA Regional Center, a scientist with the National Radio Astronomy Observatory, and lead author of this research, published today in the Astrophysical Journal.

Observations carried out using ALMA have revealed that a lengthy and thin stream of carbon monoxide is falling onto the FU Orionis star system. Interestingly, this gas does not appear to have enough fuel to sustain the current outburst. Instead, scientists believe that this accretion streamer is a remnant from a previous, much more significant feature that fell into the system.

Hales suspects that the interaction with a bigger stream of gas in the past caused the system to become unstable and trigger the brightness increase.

Astronomers used several configurations of ALMA antennas to capture the different types of emission coming from FU Orionis and study the mass flow into the star system. They also combined novel numerical methods to model the mass flow as an accretion streamer and estimate its properties.

The ALMA observations also revealed an outflow of slow-moving carbon monoxide from the star system. This gas is not associated with the gas from the most recent outburst, but it is similar to outflows observed around other protostellar objects.

"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 discs around nascent stars and the planets they eventually form," Hales explained.