A Better Understanding of Protostellar Accretion
The large outbursts seen in FU Ori and a few dozen other forming stars demonstrate that their circumstellar disks are susceptible to instabilities, though the trigger of the instability is still unclear. The increased pace disrupts the delicate balance between the stellar magnetic field and the inner edge of the disk, leading to material moving closer in and eventually touching the star’s surface.
"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," said Lynne Hillenbrand, professor at Caltech and a co-author of the paper. "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 that we saw so much extra — it was much brighter in the ultraviolet than we predicted — that was the big surprise."
The enhanced infall rate and proximity of the accretion disk to the star make FU Ori objects much brighter than a typical young stellar object. 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 significantly. "Our observations provide the strongest evidence yet that for FU Ori, the strong accretion crushes the stellar magnetosphere, so gas in the disk flows directly onto the star and shocks where it hits the stellar surface," said Prof. Herczeg.
Emission from FU Ori outbursts at optical wavelengths has been well explained with a super-heated accretion disk. However, those models predicted very little emission in the far-ultraviolet. "The Hubble data indicates a much hotter impact region than models have previously predicted," said Adolfo Carvalho, a PhD student at Caltech and the lead author of the study. "In FU Ori, the temperature is 16,000 K [nearly three times our Sun's surface temperature]. That sizzling temperature is almost twice the amount prior models have calculated. It challenges and encourages us to think of how such a jump in temperature can be explained."
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 lot of ultraviolet light.
Planet Survival Around FU Ori
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," explained Carvalho. "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, any rocky planets that are forming close to such a star."
Additional work with the Hubble UV observations is in progress. The team is carefully analyzing the various spectral emission lines from multiple elements present in the COS spectrum. This should provide further clues on FU Ori's environment, such as the kinematics of inflowing and outflowing gas within the inner region.
"A lot of these young stars are spectroscopically very rich at far ultraviolet wavelengths," reflected Hillenbrand. "A combination of Hubble's size and wavelength coverage, as well as FU Ori's fortunate circumstances, let us see further down into the engine of this fascinating star-type than ever before."
This text was adapted from a press release written by Abigail Major and Ray Villard at Space Telescope Science Institute in Baltimore, MD, USA, https://hubblesite.org/contents/news-releases/2024/news-2024-037. The observations were taken as part of General Observer program 17176. The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
Figure 1: An artist's conception of the early stages of an FU Ori outburst (credit: NASA-JPL and Caltech). The accretion flow is powerful enough that gas flows directly onto the stellar surface. The disk is superheated by the accretion, outshining the central star by a factor of 100.
Figure 2: The emission measured by HST-COS and HST-STIS (purple line and pink circles) is well explained by standard models (green line) at optical wavelengths but is 1000 times stronger than those models in the far-ultraviolet. This excess emission (pink line) is explained by emission from a 16,000 K shock where gas in the disk flows onto the stellar surface.
