Scientists decode the mysterious brightness of FU Orionis in the Orion constellation

Artist's impression of the large-scale view of FU~Ori. Credit: NRAO.

In the vast expanse of the Orion constellation, a unique double star system known as FU Orionis has puzzled astronomers since 1936.

This young star system baffled scientists when it suddenly brightened by a thousandfold, a phenomenon typically associated with dying stars, not young ones like FU Orionis.

This unusual behavior led to the creation of a new class of stars, termed FUor stars, characterized by their sudden, explosive increases in brightness, followed by a gradual dimming over several years.

Understanding why and how these young stars undergo such dramatic changes has been a significant challenge in astronomy.

Recently, astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have made a groundbreaking discovery that sheds light on this stellar mystery.

Led by Antonio Hales, deputy manager of the North American ALMA Regional Center, the team has, for the first time, observed direct evidence of the material that fuels these extraordinary eruptions.

FU Orionis has been actively consuming material from its surroundings for almost a century to sustain its luminous outburst.

The latest observations by ALMA have revealed a thin stream of carbon monoxide gas falling onto FU Orionis.

This stream is not sufficient to maintain the current outburst but is thought to be remnants from a much larger gas stream that previously fed the young star system.

Hales explains that this earlier interaction with a more substantial stream of gas likely destabilized the system, triggering the massive increase in brightness.

This finding helps demystify the process by which these eruptions occur and how young stars like FU Orionis replenish their mass.

The study, detailed in the journal The Astrophysical Journal, utilized several configurations of ALMA antennas to capture different types of emissions from FU Orionis and to monitor the flow of mass into the star system. This sophisticated approach allowed the team to model the accretion streamer’s properties accurately.

Ph.D. candidate Aashish Gupta from the European Southern Observatory (ESO), who developed the methods used to model the accretion streamer, noted the perfect alignment between the observed structure and the theoretical predictions of gas infall.

The research also revealed a slow-moving outflow of carbon monoxide gas from FU Orionis, similar to outflows observed around other young stellar objects. This insight is critical for understanding the broader processes of star and planet formation.

Sebastián Pérez of Universidad de Santiago de Chile, a co-author of the study, highlighted ALMA’s unique capability to explore a wide range of angular scales, providing a comprehensive view of the dynamics of star and planet formation.

This new understanding of FU Orionis not only resolves long-standing questions about FUor stars but also enhances our knowledge of star formation and the early life of stars.