Astronomers Uncover Surprising Findings About FU Orionis Using Hubble's Ultraviolet Observations.

This artist's concept depicts the early stages of FU Orionis' outburst, surrounded by a material disk. Using Hubble's ultraviolet capabilities, astronomers discovered that the inner disk touching the star is unexpectedly hot—16,000 kelvins—almost three times the Sun’s surface temperature, nearly double what was previously predicted.

In 1936, astronomers were baffled when the young star FU Orionis (FU Ori) suddenly became a hundred times brighter in a matter of months. Unlike an exploding star, FU Ori’s luminosity gradually declined, but it remained a star of interest for decades. Now, a team of astronomers has used NASA’s Hubble Space Telescope to delve deeper into the interaction between FU Ori’s surface and the accretion disk feeding gas to the growing star. Their groundbreaking findings challenge existing models and offer new insights into the nature of these eruptive stars.

The Hubble observations, made using the telescope's Cosmic Origins Spectrograph (COS) and Space Telescope Imaging Spectrograph (STIS) instruments, produced the first far-ultraviolet and new near-ultraviolet spectra of FU Ori. The team had aimed to examine the temperature of the accretion disk’s inner edge but were surprised to find the impact region between the disk and the star to be much hotter than previously predicted.

“We were hoping to validate the hottest part of the accretion disk model, but we were certainly not expecting what we saw,” said Lynne Hillenbrand, co-author of the study from Caltech. “The star was much brighter in the ultraviolet than we predicted — that was the big surprise.”

FU Ori is a member of a class of young, eruptive stars known as FU Orionis stars, which are part of the broader T Tauri star category. These stars are in their early formation stages and are accumulating material from their surrounding accretion disks. However, unlike classical T Tauri stars, FU Ori stars experience instability in their disks, leading to dramatic changes in the rate of material accretion. This instability causes the disk to eventually touch the star’s surface, resulting in significant increases in brightness. During outbursts, the disk even outshines the star itself.

The team’s findings revealed that the region where the disk impacts FU Ori's surface is at a scorching temperature of 16,000 kelvins, nearly three times the surface temperature of our Sun. This temperature is almost double what previous models predicted. The team proposes that this heat is generated by a shockwave produced when the disk's material impacts the star’s surface, emitting an excess of ultraviolet light.

“This sizzling temperature is nearly twice the amount previous models have calculated,” explained Adolfo Carvalho, lead author from Caltech. “It challenges and encourages us to rethink how this intense temperature could be explained.”

The implications of these findings extend beyond stellar physics and into the realm of planet formation. FU Ori’s dramatic outbursts could significantly influence the evolution of planets forming in its vicinity. While planets far from the star may survive the outbursts and inherit chemicals from the accretion disk, planets forming close to FU Ori risk being destroyed by the intense heat and radiation. Such planets could be rapidly drawn inward and either merge with the star or be vaporized in the process.

Further analysis of the Hubble data is underway, with researchers examining the spectral lines of various elements to gain deeper insights into the dynamics of gas inflows and outflows near FU Ori. These findings contribute to the growing body of knowledge about the early stages of star and planet formation.

“The Hubble data has allowed us to see further into the engine of this fascinating star-type than ever before,” said Hillenbrand. "This is a rich area of study, and with Hubble’s size and wavelength coverage, we are able to look deeper into the star’s environment than we could have ever hoped."

These new discoveries have been published in The Astrophysical Journal Letters.

The Hubble Space Telescope, a joint project of NASA and the European Space Agency (ESA), continues to make groundbreaking discoveries after more than three decades in operation. Managed by NASA's Goddard Space Flight Center, Hubble’s work is supported by the Space Telescope Science Institute and Lockheed Martin Space.

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