Researchers using NASA’s Hubble Space Telescope have discovered that the accretion disk around the young star FU Orionis is significantly hotter than previously thought, reaching temperatures nearly three times that of our Sun.
This finding challenges existing models and suggests a more complex interaction at the star-disk interface, emitting unexpected levels of ultraviolet light.
Unprecedented Heat in Young Star’s Accretion Disk
NASA’s Hubble Space Telescope has revealed surprising new details about FU Orionis (FU Ori), a young star surrounded by an unusually hot disk of material. The disk’s temperature has been measured at a scorching 16,000 Kelvin—nearly three times the surface temperature of our Sun and almost double what scientists previously estimated.
Young stars like FU Ori often experience sudden, dramatic increases in brightness caused by eruptive fits. These stars grow by pulling in material from surrounding disks and nebulas through a process known as accretion. However, the accretion disk around FU Ori is highly unstable, leading to unpredictable fluctuations in the rate at which material is drawn in.
Surprising Findings at the Stellar Interface
“We were hoping to validate the hottest part of the accretion disk model to determine its maximum temperature by measuring closer to the inner edge of the accretion disk than ever before,” says Lynne Hillenbrand, a professor of astronomy at Caltech and the second author of a new paper published in The Astrophysical Journal Letters describing the results.
“I think there was some hope that we would see something extra, like the interface between the star and its disk, but we were certainly not expecting it. The fact we saw so much extra—it was much brighter in the ultraviolet than we predicted—was the big surprise.”
To address the significant difference in temperature between past models and the recent Hubble observations, the team offers a revised interpretation of the geometry within FU Ori’s inner region: The accretion disk’s material approaches the star and once it reaches the stellar surface, a hot shock is produced, which emits a high level of ultraviolet light.
Implications for Planet Formation and Survival
Understanding the mechanisms of FU Ori’s rapid accretion process relates more broadly to ideas of planet formation and survival.
“Our revised model based on the Hubble data is not strictly bad news for planet evolution; it’s sort of a mixed bag,” explains Adolfo Carvalho, a Caltech graduate student and lead author of the study. “If the planet is far out in the disk as it’s forming, outbursts from an FU Ori object should influence what kind of chemicals the planet will ultimately inherit. But if a forming planet is very close to the star, then it’s a slightly different story. Within a couple of outbursts, any planets that are forming very close to the star can rapidly move inward and eventually merge with it. You could lose, or at least completely fry, rocky planets forming close to such a star.”
For more on this discovery, see FU Orionis Challenges Astrophysics Models.
Reference: “A Far-ultraviolet-detected Accretion Shock at the Star–Disk Boundary of FU Ori” by Adolfo S. Carvalho, Lynne A. Hillenbrand, Kevin France and Gregory J. Herczeg, 23 September 2024, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ad74eb
Never miss a breakthrough: Join the SciTechDaily newsletter.
Follow us on Google and Google News.