Most
newborn stars are gluttons, feeding on afterbirth of dust and gas long after
igniting.
Although
this accreting activity doubles stellar surface temperatures by burning up the
material, it mysteriously
softens the emission of high-energy X-rays.
"Accreting
stars have three times less X-ray emission than non-accreting stars, which
seems unusual," said Kevin Briggs, an astrophysicist at the Swiss Federal Institute of
Technology in Zurich, Switzerland.
Now Briggs
and several teams of researchers have discovered why some stars' X-ray profiles
are so thin: The nebulous surroundings of a young star absorb the extra energy
produced by falling into it.
The
discovery gives astronomers a better glimpse into the early stages of stellar
life.
Burning
filters
Briggs explained
that dust and gas surrounding young stars act like light filters on a camera,
where gas absorbs
X-rays and dust absorbs visible light.
Yet if both
materials surrounding energetic
young stars are very dense—and soak up most of the energy they
create—Briggs said the team wondered why the stars weren't fainter.
The
filters, it turns out, burn.
"The dust
is heated so much by the radiation from the star, that it is vaporized before
it can fall on the star," said Manuel Guedel, also an astrophysicist at the
Swiss Federal Institute of Technology.
As the dust
and gas still waiting to be eaten by the young stars vaporizes, Briggs
explained, they glow like hot plasma and mimic the appearance of a star's surface.
Shocking
creation
Briggs said
repetitive "shocks" of energy create young stars' X-rays, and that
there are two recipes to make them.
The first
type of shock is produced when gas and dust falls into a star and slams into its surface at nearly 671,000 mph (1,080,000 kph). "The
impact against the star's surface can produce the high-energy shock,"
Briggs said.
The second
type of X-ray shock in young stars is produced by gas and dust jettisoned
away from a star's poles.
"It
happens when fast-moving material catches up to slow-moving material and
collides," Briggs said. But nature leans toward variety with its shocking
young stars. "What we actually see is both types in these stars," he
said.
Because
stellar meals of gas and dust absorb most young stars' X-ray outputs, the teams
think the few X-rays that can be detected originate from shocks emitted from
the stars' jets.
"This
emission must come from outside the accretion streams," Guedel said. The teams looked at 400 young stars
in the constellation Taurus to uncover their findings, which are detailed in a
recent issue of the journal Astronomy and Astrophysics.
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