Puzzle of Hot Young Stars Solved


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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