In 1936, astronomers observed the young star FU Orionis (FU Ori) in the constellation Orion dramatically brighten by a hundred times over a few months, becoming 100 times brighter than the Sun. Unlike an exploding star, FU Ori’s luminosity has since declined slowly.
Recently, a team of astronomers used NASA’s Hubble Space Telescope to study the interaction between FU Ori’s surface and the accretion disk, which has been feeding gas to the star for nearly 90 years. Their findings reveal that the inner part of the accretion disk, which touches the star, is unexpectedly hot, challenging previous understanding.
The observations were made using Hubble’s COS and STIS instruments, which provide the first far-ultraviolet and new near-ultraviolet spectra of FU Ori.
Lynne Hillenbrand of Caltech in Pasadena, California, and a co-author of the paper, said, “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. There was some hope that we would see something extra, like the interface between the star and its disk, but we were not expecting it. The fact we saw so much extra — it was much brighter in the ultraviolet than we predicted — that was the big surprise.”
FU Ori is a young, eruptive star that experiences dramatic brightness changes and belongs to a subset of classical T Tauri stars. Unlike typical T Tauri stars, whose magnetic field keeps the accretion disk away from the star, the disks around FU Ori stars are more prone to instability due to their large mass, interactions with binary companions, or infalling material.
This instability causes dramatic changes in the mass accretion rate, disrupting the balance between the star’s magnetic field and the disk’s inner edge, eventually leading to material touching the star’s surface. This increased accretion and closer disk proximity make FU Ori stars much brighter than typical T Tauri stars, with the disk even outshining the star during outbursts.
The rapid orbiting of disk material near the star causes it to slow down and heat up significantly at the point of impact.
Adolfo Carvalho of Caltech, the study’s lead author, said, “The Hubble data indicates a much hotter impact region than models have previously predicted. FU Ori’s temperature is 16,000 kelvins [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 explain the temperature discrepancy between past models and recent Hubble observations, the team proposes a revised interpretation of the geometry in FU Ori’s inner region. According to this new view, the material from the accretion disk moves closer to the star, and when it reaches the stellar surface, it produces a hot shock. This shock generates significant ultraviolet light, which helps account for the observed temperatures.
Understanding FU Ori’s rapid accretion process has broader implications for planet formation and survival. The revised model based on Hubble data suggests that if a planet is forming far from the star, outbursts from an FU Ori object can influence its chemical composition.
However, if a planet forms very close to the star, it may be in danger. Within a few outbursts, such planets could move inward and merge with the star, potentially destroying or severely damaging rocky planets.
The team continues to analyze the Hubble UV observations, focusing on spectral emission lines from various elements in the COS spectrum. This will provide further insights into the kinematics of the inflowing and outflowing gas in FU Ori’s inner region.
According to Hillenbrand, the combination of Hubble’s size, wavelength coverage, and FU Ori’s unique characteristics allows scientists to study the inner workings of this star type more deeply than ever before.
Journal Reference:
- Adolfo S. Carvalho, Lynne A. Hillenbrand et al. A Far-ultraviolet-detected Accretion Shock at the Star–Disk Boundary of FU Ori. The Astrophysical Journal Letters. DOI 10.3847/2041-8213/ad74eb