Astronomers: Asteroid Collisions May Account For Star's Odd Appearance

http://www.napa.ufl.edu/2005news/lopsidedstar.htm

ASTRONOMERS: ASTEROID COLLISIONS MAY ACCOUNT FOR STAR'S ODD APPEARANCE
University of Florida
January 12, 2005

GAINESVILLE, Fla. --- The recent
collision of two huge asteroids or tiny planets may be the cause of the
mysterious lopsided appearance of the most famous of the universe's
planet-forming stars, a team of astronomers says.

Relying on observations from the Gemini South telescope in Chile, the
University of Florida-led team has concluded that differences in
brightness in the dust disc surrounding a star known as Beta Pictoris
stem from an extra bright clump on one side of the disc. This clump,
the
astronomers say, is composed of dust particles that are consistently
smaller than particles elsewhere in the disc - likely evidence of a
collision of two massive asteroids or tiny developing planets known as
planetismals that may have occurred as recently as in the past few
decades.

An article about the discovery is set to appear Jan. 13 in the journal
Nature.

"What we're proposing is that a planetesimal - either a very small
planet or a very large asteroid -- has collided with another similar
object and has been catastrophically destroyed," said Charlie Telesco,
a
UF astronomy professor and the paper's lead author. "It's a cloud now,
but what we're proposing is that this cloud represents the debris of a
major collision."

The findings are of interest because they suggest a new explanation for
a phenomenon -- asymmetries or lopsided appearances in star dust discs
--- that has long puzzled astronomers, Telesco said. The results also
continue to help refine the evolving science of planet-finding, an
endeavor that has turned up more than 100 planets outside our solar
system since the first was discovered in 1995.

Stars are thought to form when gravity causes a rotating cloud of gas
to
contract. Before the actual star is formed, the gas collapses into a
rotating disk of gas and dust particles ranging in size from tiny
grains
to household-sized dust to rocks and boulders. Astronomers had long
predicted that some of this material may coagulate into planets as it
rotates around the core, but Beta Pictoris, first detected by the
Infrared Astronomy Satellite in 1983, was the first such
"circumstellar"
star to be imaged.

Beta Pictoris is about 63 light years from Earth in the southern
constellation known as Pictor, or Painter's Easel. It barely clears the
horizon on the southern edges of the Northern Hemisphere, where it is
seen most easily from Hawaii.

Like other planet-forming stars, Beta Pictoris, which is between 10
million and 20 million years old, is extremely young by stellar
standards, with mature stars living billions of years. Also like some
other young stars, it has an attribute that has long proved puzzling to
astronomers: One side, or "wing," of the star's 200-billion-mile
diameter dust disc is both brighter and longer than the other.

Some astronomers theorized that this anomaly was caused by the presence
of a large planet orbiting the star. But the UF-led team came to a
different conclusion after observing Beta Pictoris during six nights in
December 2003 and last January using the Gemini telescope. The
telescope
had been specially equipped with a UF-designed and -built observational
camera called the Thermal Region Camera and Spectograph, or T-ReCS,
according to Telesco.

T-ReCS allows astronomers to detect faint sources of thermal or
infrared
radiation by isolating them from the far more powerful and more obvious
radiation generated by the Earth's atmosphere, the telescope and the
star itself.

"We're able to see sources that are at least a million times fainter
than the background," Telesco said. "It's like trying to detect a match
when you're actually holding the match up to the sun."

What Telesco characterized as "the most complete and the best
resolution
imaging at multiple wavelengths" of the star revealed that the wing's
brightness stemmed from a "knot" of emissions, or clump. Further
examination showed this clump contained a higher concentration of
smaller, finer dust particles than elsewhere, suggesting a violent and
recent collision of asteroids or tiny planets.

"Many of us remember pounding chalk dust out of erasers at school,"
said
Scott Fisher, an astronomer at Gemini South and a co-author of the
Nature paper. "After you sneeze a few times, you open a window and the
fine dust blows away. In Beta Pictoris, the radiation from the star
should blow away the fine particles from the collision quite rapidly.
The fact that we still see them in our observations means that the
collision probably happened in the past 100 years or so."

An alternative explanation of the clump may be the "collisional
grinding" of two planets located closely together in orbit, Telesco
said. "Over time, the planets bang into each other, and when they do
they actually produce debris," he said.

The findings suggest a possible explanation for other observed lopsided
discs, Telesco said. They also may help astronomers weed out planets
from other possible sources of brightness.

"One of the problems for astronomers is if there are clumps in the disc
associated with planetismals, it's hard to tell the difference between
those clumps and a planet," he said. "So we're hoping to use these
results to understand how we can distinguish these structures from
planets until the time comes when we have sensitivity to see the planet
itself."

Besides Telesco, the other UF authors of the paper are Stan Dermott,
professor and chairman of the astronomy department; Thomas Kehoe, an
assistant scientist; Steven Novotny, a recent UF graduate; Naibl
Mari?as, a graduate student; James Radomski, a postdoctoral research
associate; and Christopher Packham, an assistant scientist.

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