Free Astronomy Magazine January-February 2025

JANUARY-FEBRUARY 2025 A rtist’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 enve- lope, and the CO gas swept up by the shock is what the new ALMA re- vealed. [NSF/NRAO/S. Dagnello] Z oom into the FU Ori binary sys- tem and the newly discovered accretion streamer. This artist’s im- pression shows the newly discov- ered streamer constantly feeding mass from the envelope into the bi- nary system. [NSF/NRAO/S. Dagnello] ion, or infalling material. Such in- stability means the mass accretion rate can change dramatically. The increased pace disrupts the delicate balance between the stellar mag- netic field and the inner edge of the disk, leading to material mov- ing closer in and eventually touch- ing the star’s surface. The enhanced infall rate and prox- imity of the accretion disk to the star make FU Ori objects much brighter than a typical T Tauri star. In fact, during an outburst, the star itself is outshined by the disk. Furthermore, the disk material is orbiting rapidly as it approaches the star, much faster than the rotation rate of the stellar surface. This means that there should be a region where the disk impacts the star and the material slows down and heats up signifi- cantly. “The Hubble data indicates a much hotter impact region than models have previously predicted,” said Adolfo Carvalho of Caltech and lead author of the study. “In FU Ori, the temperature is 16,000 kelvins [nearly three times our Sun’s surface tem- perature]. That sizzling temperature is almost twice the amount prior models have calculated. It chal- lenges and encourages us to think of how such a jump in temperature can be explained.” To address the significant difference in temperature between past mod- els and the recent Hubble observa- tions, the team offers a revised in- terpretation of the geometry within FU Ori’s inner region: The accretion disk’s material approaches the star and once it reaches the stellar sur- face, a hot shock is produced, which