Gluttonous Star May Hold Clues to Planet Formation

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Gluttonous Star May Hold Clues to Planet Formation
Jet Propulsion Laboratory
June 14, 2016

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

For more information about SOFIA, visit:

http://www.nasa.gov/sofia

http://www.dlr.de/en/sofia

News Media Contact
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabet...@jpl.nasa.gov

2016-151