YOUNG
STAR BLASTS ITS WAY OUT OF STELLAR NURSERY A
young star is apparently blasting high-speed gas jets from its poles, carving
a void through the dense molecular cloud that was its birthplace, according to
an international team of astronomers. The observation indicates that the jets
are more common and last longer than supposed. The
team observed "SU Aurigae," located about 480 light-years from Earth
in the direction of the constellation Auriga, with the Space Telescope Imaging
Spectrometer (STIS) on board the Hubble Space Telescope. A coronagraph was used
to eclipse the star so the much fainter surrounding gas could be seen. (One light-year
is almost six trillion miles, or 9.6 trillion kilometers). "If
these features are indeed gas jets from SU Aurigae, then they are more common
around intermediate-mass stars than previously believed," said Dr. Carol
Grady, an astronomer with Eureka Scientific and the National Optical Astronomy
Observatory who is stationed at NASA's Goddard Space Flight Center, Greenbelt,
Md. "We only have 11 coronagraphic images of stars like SU Aurigae, and it's
already the second one we've seen with features that look like gas jets. Unless
we are incredibly lucky, gas jets are likely a relatively common feature of young
stars, regardless of their mass," said Grady. "Also,
the jets were once thought to last for only the first 100,000 years or so of a
star's lifetime, but SU Aurigae is about four million years old, so we are learning
that jets can persist for a lot longer than we thought." Grady is presenting
a poster about this research today at the American Astronomical Society meeting
in Washington, D.C. The
team observed features resembling a jet of gas extending about 110 billion miles
in front of SU Aurigae, and a fainter jet about 100 billion miles long behind
the star, directed into a vast, dark molecular cloud that provided the matter
for the star's -more
-2- formation.
Additionally, the team saw a 170 billion-mile wide void in the molecular cloud
around SU Aurigae. The void is filled with thin gas that reflects light from the
star, making it visible in the STIS coronagraphic image. According to the team,
the void could have been carved by the gas jets and radiation pressure from the
star. Astronomers
believe gas jets are emitted by stars which are still forming that also have strong
magnetic fields. The star's gravity pulls gas from its parent cloud, while the
star's magnetic field, which is twisted into rope-like structures at the poles
due to the star's rotation, expels some of the infalling gas as high-speed polar
jets. Gas
jets are commonly observed around young stars with the mass of the Sun or less,
but SU Aurigae has nearly twice the Sun's mass. The common presence of jets around
intermediate-mass stars like SU Aurigae is a surprise because such stars typically
do not have a strong, global magnetic field required to direct some of the infalling
gas into a jet. This is because their internal structure is different from lower-mass
stars; they lack a turbulent layer near their surface that generates a strong,
global magnetic field. "It
may be that a young, intermediate-mass star has a similar structure to a solar-mass
star, which allows it to generate a strong magnetic field. However, when the intermediate-mass
star matures and starts shining by hydrogen fusion in its core, its structure
changes and the powerful magnetic field of its youth gradually fades," said
Grady. Astronomers
were unsure how long infalling matter persists to feed the jets; this observation
indicates that, in some cases, the jets survive for millions of years. "Some
astronomers want well-defined stages for starbirth, but reality, as usual, is
more messy," said Grady. "We are finding that the timing of stellar
birth phases depends a lot on the star's environment. If a star has nearby companion
stars, or if it starts forming planets in its disk, these objects can clear away
gas that feeds the jets, terminating them more quickly than an isolated star without
planets." SU
Aurigae's lifetime is estimated to be less than a billion years, making its current
age equivalent to about three and a half months of a 70-year human lifespan. Support
for this research was provided by NASA through a grant from the Space Telescope
Science Institute, which is operated by the AURA Inc., under NASA contract NAS
5-26555. Back
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