New results from NASA's Hubble Space Telescope reveal that a young star, FU Orionis (FU Ori), is circled by a disk of material that is much hotter than expected—16,000 Kelvin, or nearly three times our Sun's surface temperature. That sizzling temperature is nearly twice as hot as previously believed.
Young stars like FU Ori go through eruptive fits that lead to dramatic changes in brightness. These growing stars accumulate mass by tugging on material from surrounding disks and nebulas, a process called accretion. The accretion disk around FU Ori is susceptible to instabilities, which means the accretion rate can change dramatically.
"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.
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 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."
Read the full story from the Hubble Space Telescope website.
The study titled "An FUV-detected Accretion Shock at the Star-Disk Boundary of FU Ori," was funded by NASA.
Written by Whitney Clavin