XMM-Newton reveals a magnetic surprise (Forwarded)

ESA News
http://www.esa.int

22 February 2007

XMM-Newton reveals a magnetic surprise

ESA's X-ray observatory XMM-Newton has revealed evidence for a magnetic
field in space where astronomers never expected to find one. The magnetic
field surrounds a young star called AB Aurigae and provides a possible
solution to a twenty-year-old puzzle.

At 2.7 times the mass of the Sun, AB Aurigae is one of the most massive
stars in the Taurus-Auriga star-forming cloud. Although amongst nearly 400
smaller stars, its ultraviolet radiation plays a key role in shaping the
cloud. Its massive status puts it in a class known as Herbig stars, named
after their discoverer George Herbig.

As part of a large programme to survey Taurus-Auriga at X-ray wavelengths,
XMM-Newton systematically targeted AB Aurigae and the other young stars in
this region, using its European Photon Imaging Camera (EPIC). AB Aurigae
stood out brightly in the image, indicating that it was releasing X-rays.

X-rays are expected to come from young stars with strong magnetic fields
but computer calculations have repeatedly suggested that Herbig stars do
not have the correct internal conditions to generate an appreciable
magnetic field. Yet for twenty years, astronomers have been detecting
X-ray emission from them.

Where could the X-rays be coming from? Some astronomers suggested that
Herbig stars could have a smaller companion star in orbit around them and
the X-rays are coming from the companion.

However, when an international team led by Manuel Güdel and his graduate
student Alessandra Telleschi, of the Paul Scherrer Institut, Switzerland,
analysed the AB Aurigae data, they found that the temperature of the gas
producing the X-rays lay between one and five million degrees centigrade.
"That was suspiciously low," Güdel says. Young sun-like stars possess
gaseous atmospheres that are heated to 10 million degrees and higher, by
their magnetic field.

Güdel and his team found another clue that the X-rays must be coming from
AB Aurigae itself: the X-rays varied in intensity every 42 hours. This is
a magic number for the star because astronomers know that the optical and
ultraviolet light from AB Aurigae also varies by this same amount.
"Finding the same periodicity confirms that the X-rays are coming from AB
Aurigae and not from a companion star," says Güdel. But how are they
generated?

To search for an explanation Telleschi and colleagues looked at
high-resolution data of AB Aurigae taken with the orbiting observatory's
Reflection Grating Spectrometers.

In this data they looked for a spectral fingerprint that would tell them
how far above the star¹s surface the X-ray-emitting gas was located.

To their surprise, they found that the X-rays were coming from high above
the star. They had expected them to be much closer to the surface. X-rays
high above the surface means that gas given off by the star, called the
stellar wind, from two different hemispheres is probably being guided
together into a collision. And the only thing that could do that was a
magnetic field. It would not be a strong magnetic field, but it had to be
a magnetic field nonetheless.

Luckily, a group of astronomers who had developed a magnetic field model
of this kind for another class of star also worked in the Taurus-Auriga
observing team. So it was easy for them to contribute their expertise.

Working with them, Telleschi, Güdel and their colleagues now propose that,
as the vast pocket of gas collapsed to become AB Aurigae, it pulled with
it part of the magnetic field that threaded that region of space. This
field is now trapped inside the star and funnels the stellar winds
together. Winds from the two hemispheres thus collide to create the
X-rays.

It is a neat explanation for a twenty-year mystery but, at the moment,
Güdel and colleagues do not know whether this is applicable to other
Herbig stars. "That's the important question," Güdel says. To resolve it,
high-resolution spectra of other Herbig stars will have to be taken.

Note for editors

"The first high-resolution X-ray spectrum of a Herbig Star: AB Aurigae,"
by Alessandra Telleschi, Manuel Güdel, Kevin Briggs, Stephen Skinner, Marc
Audard, and Elena Franciosini, will be published in a forthcoming issue of
Astronomy and Astrophysics.

For more information:

Manuel Güdel, Paul Scherrer Institut, Switzerland
Email: guedel @ astro.phys.ethz.ch

Norbert Schartel, ESA XMM-Newton Project Scientist
Email: norbert.schartel @ sciops.esa.int

[NOTE: Images and weblinks supporting this release are available at
http://www.esa.int/esaCP/SEM01WBE8YE_index_1.html ]


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