A star in the Orion constellation suddenly became 100 times brighter than the Sun nearly 100 years ago, and using the Hubble Space Telescope, astronomers may now have found the reason why.
The story begins in 1936, when astronomers noticed young star FU Orionis (FU Ori) had become a hundred times brighter in just a few months.
At its peak brightness, FU Orionis was 100 times brighter than the Sun, and has faded in brightness only slowly since that time.
More Hubble discoveries
Investigating FU Orionis's extreme brightness
What could have caused FU Orionis to suddly become so bright?
Astronomers used the Hubble Space Telescope to get a closer look and delve into the mystery.
The solution, they say, relates to the accretion disk that's left surrounding a young star shortly after its formation.
This disk of gas and dust is made up of the leftover ingredients from which the star formed, and contains materials out of which planets may form, the way they did around our Sun.
The Hubble Space Telescope's ultraviolet vision gave the team a good look at the interaction between FU Orionis's surface and accretion disk.
This disk has been dumping gas onto the star for about 90 years. What's more, it's scorchingly hot, which is unexpected.
"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 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 solution at last?
FU Ori is one of a class of young stars that experience dramatic changes in brightness.
Typically, T Tauri stars like FU Orionis do not directly touch their accretion disks because the disk is kept at bay by the outward push of the star's magnetic field.
But there is a subset of T Tauri stars, known as 'FU Ori objects', that do undergo extreme changes in brightness, and for a few possible reasons.
They might have accretion disks that are enormous, relative to their star.
Or they may have a binary companion star that interacts with them.
Another way in which these stars might display changes in luminosity is material falling inwards and heating up, the same process that causes black holes to shine brightly.
FU Ori objects' accretion disks can even outshine the star itself.
In cases where the change in brightness is caused by the accretion disk coming into direct contact with the star, the disk material is orbiting rapidly as it approaches the star.
It hits the surface of the star, slows down and heats up.
"The Hubble data indicates a much hotter impact region than models have previously predicted," says 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."
Could FU Orionis have planets?
We know that planets can form within accretion disks, but could any planets have a hope of forming within the disk around such a young, unstable star as FU Orionis?
"Our revised model based on the Hubble data is not strictly bad news for planet evolution, it's sort of a mixed bag," says 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."
The team are carrying out more observations with Hubble and analysing light from the star to understand more about its chemical makeup. This is known as spectroscopy.
"A lot of these young stars are spectroscopically very rich at far ultraviolet wavelengths," says 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."
Read the full paper at stsci-opo.org/STScI-01JD2FJFVWT314PXEHG17CXSGV.pdf
