Hubble Finds Stars That 'Go Ballistic'

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Whitney Clavin 818-354-4673
Jet Propulsion Laboratory
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Donna Weaver /Ray Villard 410-338-4493 / 410-338-4514
Space Telescope Science Institute, Baltimore, Md.
dweaver@stsci.edu / villard@stsci.edu

NEWS RELEASE: 2009-002 Jan. 7, 2009

Hubble Finds Stars That 'Go Ballistic'

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 .

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