http://www.jpl.nasa.gov/news/news.cfm?release=2009-002

Hubble Finds Stars That 'Go Ballistic'
Jet Propulsion Laboratory
January 07, 2009

Some stars go ballistic, racing through interstellar space like
bullets
and tearing through clouds of gas.

Images from NASA's Hubble Space Telescope, taken by Raghvendra Sahai
of
NASA's Jet Propulsion Laboratory, Pasadena, Calif., and colleagues
reveal 14 of these young, runaway stars.

The stars are plowing through regions of dense interstellar gas,
creating brilliant arrowhead structures and trailing tails of glowing
gas. These arrowheads, or bow shocks, form when the stars' powerful
stellar winds, streams of matter flowing from the stars, slam into
surrounding dense gas. The phenomenon is similar to that seen when a
speeding boat pushes through water on a lake.

"We think we have found a new class of bright, high-velocity stellar
interlopers," said Sahai. "Finding these stars is a complete surprise
because we were not looking for them. When I first saw the images, I
said, 'Wow. This is like a bullet speeding through the interstellar
medium.' Hubble's sharp 'eye' reveals the structure and shape of these
bow shocks."

The astronomers can only estimate the ages, masses and velocities of
these renegade stars. The stars appear to be young -- just millions of
years old. Their ages are based partly on their strong stellar winds.

Most stars produce powerful winds either when they are very young or
very old. Only very massive stars greater than 10 times the sun's mass
have stellar winds throughout their lifetimes.

But the objects observed by Hubble are not very massive because they
do
not have glowing clouds of ionized gas around them. They are
medium-sized stars that are a few to eight times more massive than the
sun. The stars are not old because the shapes of the nebulae around
aging, dying stars are very different, and old stars are almost never
found near dense interstellar clouds.

Depending on their distance from Earth, the bullet-nosed bow shocks
could be 100 billion to a trillion miles wide (the equivalent of 17 to
170 solar system diameters, measured out to Neptune's orbit). The bow
shocks indicate that the stars are traveling fast, more than 180,000
kilometers an hour (more than 112,000 miles an hour) with respect to
the
dense gas they are plowing through, which is roughly five times faster
than typical young stars.

"The high-speed stars were likely kicked out of their homes, which
were
probably massive star clusters," Sahai said.

There are two possible ways this stellar expulsion could have
happened.
One way is if one star in a binary system exploded as a supernova and
the partner got kicked out. Another scenario is a collision between
two
binary-star systems or a binary system and a third star. One or more
of
these stars could have picked up energy from the interaction and
escaped
the cluster.

Assuming their youthful phase lasts only a million years and they are
moving at roughly 180,000 kilometers an hour (about 112,000 mph), the
stars have traveled about 160 light-years.

Runaway stars have been seen before. The joint European-NASA Infrared
Astronomical Satellite, which performed an all-sky infrared survey in
1983, spied a few similar-looking objects. The first observation of
these objects was in the late 1980s. But those stars produced much
larger bow shocks than the stars in the Hubble study, suggesting that
they are more massive stars with more powerful stellar winds.

"The stars in our study are likely the lower-mass and/or lower-speed
counterparts to the massive stars with bow shocks detected by the
Infrared Astronomical Satellite," Sahai explained. "We think the
massive
runaway stars observed before were just the tip of the iceberg. The
stars seen with Hubble may represent the bulk of the population, both
because many more lower-mass stars inhabit the universe than higher-
mass
stars, and because a much larger number are subject to modest speed
kicks."

Sahai presented his results at the American Astronomical Society
meeting
in Long Beach, Calif. The science team also includes M. Morris of the
University of California, Los Angeles; M. Claussen of the National
Radio
Astronomy Observatory in Socorro, N.M.; and R. Ainsworth of the
University of Tennessee in Knoxville.

JPL is managed by the California Institute of Technology, Pasadena,
for
NASA. More information is at http://www.nasa.gov/hubble and
www.jpl.nasa.gov .

Media contacts: Whitney Clavin 818-354-4673
Jet Propulsion Laboratory
whitney.clavin{at}jpl.nasa.gov

Donna Weaver /Ray Villard 410-338-4493 / 410-338-4514
Space Telescope Science Institute, Baltimore, Md.
dweaver{at}stsci.edu / villard{at}stsci.edu

2009-002
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