In 1936, astronomers saw a puzzling event in the constellation Orion: the young star FU Orionis (FU Ori) became a hundred times brighter in a matter of months. At its peak, FU Ori was intrinsically 100 times brighter than our Sun. Unlike an exploding star though, it has declined in luminosity only languidly since then.
Now, a team of astronomers has
wielded NASA's Hubble
Space Telescope's ultraviolet capabilities to learn more about the interaction between FU
Ori's stellar surface and the accretion disk that has been dumping gas onto the
growing star for nearly 90 years. They find that the inner disk touching the
star is extraordinarily hot — which challenges conventional wisdom.
The observations were made with the
telescope's COS (Cosmic Origins Spectrograph) and STIS (Space Telescope Imaging Spectrograph) instruments. The data includes the first
far-ultraviolet and new near-ultraviolet spectra of FU Ori.
"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 of Caltech in Pasadena, California,
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 we saw so much extra — it was much
brighter in the ultraviolet than we predicted — that was the big
surprise."
A Better Understanding of
Stellar Accretion
Originally deemed to be a unique
case among stars, FU Ori exemplifies a class of young, eruptive stars that
undergo dramatic changes in brightness. These objects are a subset of classical
T Tauri stars, which are newly forming stars that are building up by accreting
material from their disk and the surrounding nebula. In classical T Tauri
stars, the disk does not touch the star directly because it is restricted by
the outward pressure of the star's magnetic field.
The accretion disks around FU Ori
objects, however, are susceptible to instabilities due to their enormous mass
relative to the central star, interactions with a binary companion, or
infalling material. Such instability means the mass accretion rate can change
dramatically. 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.
This is an artist's concept of the early stages of the
young star FU Orionis (FU Ori) outburst, surrounded by a disk of material. A
team of astronomers has used the Hubble Space Telescope's ultraviolet
capabilities to learn more about the interaction between FU Ori's stellar
surface and the accretion disk that has been dumping gas onto the growing star
for nearly 90 years. They found that the inner disk, touching the star, is much
hotter than expected—16,000 kelvins—nearly three times our Sun's surface temperature.
That sizzling temperature is nearly twice as hot as previously believed.
NASA-JPL, Caltech
The enhanced infall rate and
proximity of the accretion disk to the star make FU Ori objects much brighter
than a typical T Tauri star. 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.
"The Hubble data indicates a
much hotter impact region than models have previously predicted," said
Adolfo Carvalho of Caltech and lead author of the study. "In FU Ori, the
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 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, rocky planets 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, its 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."
These findings have been published
in The Astrophysical Journal Letters.
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.
Source: NASA's Hubble Finds Sizzling Details About Young Star FU Orionis - NASA Science
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