Credits: NASA-JPL, Caltech


In the constellation Orion, astronomers saw in 1936 something quite extraordinary: the young star FU Orionis (FU Ori) brightened by 100 times within a few months. Unlike an exploding star, FU Ori has been gradually fading since then. Now, some 90 years later, the NASA Hubble Space Telescope has made new discoveries about this interesting star with its ultraviolet capabilities.

Astronomers aimed to study the interaction between FU Ori’s surface and its accretion disk—the massive structure of gas spiraling onto the star. Using Hubble's Cosmic Origins Spectrograph (COS) and Space Telescope Imaging Spectrograph (STIS), they captured unprecedented ultraviolet spectra of FU Ori.

Data revealed a surprise-the region where the accretion disk meets the star is much hotter than expected, reaching temperatures of 16,000 kelvins - nearly three times hotter than our Sun's surface and double what previous models predicted.

"We were hoping to validate the disk's hottest region," said Lynne Hillenbrand of Caltech. "But what we found was much brighter in the ultraviolet than anticipated. This unexpected result challenges our understanding of these processes."

FU Ori is part of an extremely small fraction of eruptive young stars, being a subgroup of T Tauri stars. Unlike most T Tauri stars, whose magnetic fields hold accretion disks at bay, FU Ori is quite different. Either instabilities in its dense disk, interaction with a companion star that could be a binary, or inflowing material allow the disk to contact the star directly.

That interaction drives accretion and makes FU Ori much brighter than a typical T Tauri star. As the disk material approaches the star, it heats up and forms a hot shock region, explaining the intense ultraviolet radiation measured by Hubble.


Implications for Planet Formation

The high activity around FU Ori could affect the formation and survival of planets. Outbursts from the star can change the chemical composition of forming planets in its outer disk. However, for planets closer to the star, these events can be catastrophic.

"If a forming planet is too close, repeated outbursts could cause it to spiral into the star or burn up entirely," said Adolfo Carvalho, lead author of the study. "This adds complexity to how we understand planet evolution around such stars."

The team is now examining spectral emission lines to understand the dynamics of inflowing and outflowing gas near FU Ori. These results will open up further insight into the environment around eruptive stars. 

Hubble observations are part of General Observer Program 17176, emphasizing once again the value of the telescope to astrophysics. With over three decades of discoveries, Hubble remains an incomparable treasure for unraveling the secrets of the universe.

As Hillenbrand remarked, "Hubble lets us see further into the engine of these fascinating stars than ever before."

The complete study appears in "The Astrophysical Journal Letters"