In 1936, the young star FU Orionis began gobbling material from its
surrounding disk of gas and dust with a sudden voraciousness. During a
three-month binge, as matter turned into energy, the star became 100
times brighter, heating the disk around it to temperatures of up to
12,000 degrees Fahrenheit (7,000 Kelvin). FU Orionis is still devouring
gas to this day, although not as quickly.
This brightening is the most extreme event of its kind that has been
confirmed around a star the size of the sun, and may have implications
for how stars and planets form. The intense baking of the star's
surrounding disk likely changed its chemistry, permanently altering
material that could one day turn into planets.
"By studying FU Orionis, we're seeing the absolute baby years of a
solar system," said Joel Green, a project scientist at the Space
Telescope Science Institute, Baltimore, Maryland. "Our own sun may have
gone through a similar brightening, which would have been a crucial step
in the formation of Earth and other planets in our solar system."
Visible light observations of FU Orionis, which is about 1,500
light-years away from Earth in the constellation Orion, have shown
astronomers that the star's extreme brightness began slowly fading after
its initial 1936 burst. But Green and colleagues wanted to know more
about the relationship between the star and surrounding disk. Is the
star still gorging on it? Is its composition changing? When will the
star's brightness return to pre-outburst levels?
To answer these questions, scientists needed to observe the star’s
brightness at infrared wavelengths, which are longer than the human eye
can see and provide temperature measurements.
Green and his team compared infrared data obtained in 2016 using the Stratospheric Observatory for Infrared Astronomy, SOFIA, to observations made with NASA's Spitzer Space Telescope
in 2004.
SOFIA, the world's largest airborne observatory, is jointly
operated by NASA and the German Aerospace Center and provides
observations at wavelengths no longer attainable by Spitzer. The SOFIA
data were taken using the FORCAST instrument (Faint Object infrared
Camera for the SOFIA Telescope).
"By combining data from the two telescopes collected over a 12-year
interval, we were able to gain a unique perspective on the star's
behavior over time," Green said. He presented the results at the
American Astronomical Society meeting in San Diego, this week.
Using these infrared observations and other historical data,
researchers found that FU Orionis had continued its ravenous snacking
after the initial brightening event: The star has eaten the equivalent
of 18 Jupiters in the last 80 years.
The recent measurements provided by SOFIA inform researchers that the
total amount of visible and infrared light energy coming out of the FU
Orionis system decreased by about 13 percent over the 12 years since the
Spitzer observations. Researchers determined that this decrease is
caused by dimming of the star at short infrared wavelengths, but not at
longer wavelengths. That means up to 13 percent of the hottest material
of the disk has disappeared, while colder material has stayed intact.
"A decrease in the hottest gas means that the star is eating the
innermost part of the disk, but the rest of the disk has essentially not
changed in the last 12 years," Green said. "This result is consistent
with computer models, but for the first time we are able to confirm the
theory with observations."
Astronomers predict, partly based on the new results, that FU Orionis
will run out of hot material to nosh on within the next few hundred
years. At that point, the star will return to the state it was in before
the dramatic 1936 brightening event. Scientists are unsure what the
star was like before or what set off the feeding frenzy.
"The material falling into the star is like water from a hose that's
slowly being pinched off," Green said.
"Eventually the water will stop."
If our sun had a brightening event like FU Orionis did in 1936, this
could explain why certain elements are more abundant on Mars than on
Earth. A sudden 100-fold brightening would have altered the chemical
composition of material close to the star, but not as much farther from
it. Because Mars formed farther from the sun, its component material
would not have been heated up as much as Earth's was.
At a few hundred thousand years old, FU Orionis is a toddler in the
typical lifespan of a star. The 80 years of brightening and fading since
1936 represent only a tiny fraction of the star's life so far, but
these changes happened to occur at a time when astronomers could
observe.
"It's amazing that an entire protoplanetary disk can change on such a
short timescale, within a human lifetime," said Luisa Rebull, study
co-author and research scientist at the Infrared Processing and Analysis
Center (IPAC), based at Caltech, Pasadena, California.
Green plans to gain more insight into the FU Orionis feeding phenomenon with NASA's James Webb Space Telescope,
which will launch in 2018. SOFIA has mid-infrared high-resolution
spectrometers and far-infrared science instrumentation that complement
Webb’s planned near- and mid-infrared capabilities.
Spitzer is expected
to continue exploring the universe in infrared light, and enabling
groundbreaking scientific investigations, into early 2019.
NASA's Jet Propulsion Laboratory, Pasadena, California, manages the
Spitzer Space Telescope mission for NASA. Science operations are
conducted at the Spitzer Science Center at Caltech. Spacecraft
operations are based at Lockheed Martin Space Systems Company,
Littleton, Colorado. Data are archived at the Infrared Science Archive
housed at IPAC at Caltech. Caltech manages JPL for NASA.
SOFIA is a joint project of NASA and the German Aerospace Center
(DLR). The aircraft is based at NASA Armstrong Flight Research Center's
facility in Palmdale, California. NASA's Ames Research Center in Moffett
Field, California, manages the SOFIA science and mission operations in
cooperation with the Universities Space Research Association (USRA)
headquartered in Columbia, Maryland, and the German SOFIA Institute
(DSI) at the University of Stuttgart.
For more information about Spitzer, visit: http://www.nasa.gov/spitzer - http://spitzer.caltech.edu
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov
Editor: Tony Greicius
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov
Editor: Tony Greicius
Source: NASA/Spitzer Telescope
