Puzzle of Hot Young Stars Solved

Puzzle of Hot Young Stars Solved
The Taurus Molecular Cloud in the infrared, revealing the vast star-forming regions nearest to Earth. The cloud contains over 400 young stars. Credits: Five College Radio Astronomy Observatory/Gopal Narayanan/Mark Heyer

Mostnewborn stars are gluttons, feeding on afterbirth of dust and gas long afterigniting.

Althoughthis accreting activity doubles stellar surface temperatures by burning up thematerial, it mysteriouslysoftens the emission of high-energy X-rays.

"Accretingstars have three times less X-ray emission than non-accreting stars, whichseems unusual," said Kevin Briggs, an astrophysicist at the Swiss Federal Institute ofTechnology in Zurich, Switzerland.

Now Briggsand several teams of researchers have discovered why some stars' X-ray profilesare so thin: The nebulous surroundings of a young star absorb the extra energyproduced by falling into it.

Thediscovery gives astronomers a better glimpse into the early stages of stellarlife.

Burningfilters

Briggs explainedthat dust and gas surrounding young stars act like light filters on a camera,where gas absorbsX-rays and dust absorbs visible light.

Yet if bothmaterials surrounding energeticyoung stars are very dense?and soak up most of the energy theycreate?Briggs said the team wondered why the stars weren't fainter.

Thefilters, it turns out, burn.

?The dustis heated so much by the radiation from the star, that it is vaporized beforeit can fall on the star,? said Manuel Guedel, also an astrophysicist at theSwiss Federal Institute of Technology.

As the dustand gas still waiting to be eaten by the young stars vaporizes, Briggsexplained, they glow like hot plasma and mimic the appearance of a star's surface.

Shockingcreation

Briggs saidrepetitive "shocks" of energy create young stars' X-rays, and thatthere are two recipes to make them.

The firsttype 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). "Theimpact against the star?s surface can produce the high-energy shock,"Briggs said.

The secondtype of X-ray shock in young stars is produced by gas and dust jettisonedaway from a star's poles.

"Ithappens when fast-moving material catches up to slow-moving material andcollides," Briggs said. But nature leans toward variety with its shockingyoung stars. "What we actually see is both types in these stars," hesaid.

Becausestellar meals of gas and dust absorb most young stars' X-ray outputs, the teamsthink the few X-rays that can be detected originate from shocks emitted fromthe stars' jets.

"Thisemission must come from outside the accretion streams," Guedel said. The teams looked at 400 young starsin the constellation Taurus to uncover their findings, which are detailed in arecent issue of the journal Astronomy and Astrophysics.

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