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WAVES IN STELLAR ATMOSPHERES

March 28, 2005

ROYAL ASTRONOMICAL SOCIETY PRESS NOTICE:

WAVES IN STELLAR ATMOSPHERES

UK scientists have made the first X-ray observation of wave-like variations in a flare that erupted on a distant dwarf star. By studying the fluctuations in the flare, the team has been able to calculate that the loop of superhot gas associated with the eruption stretched over a distance of 250,000 km - 20 times the diameter of the Earth.

The discovery is based upon many years of studying the coronas - the searingly hot outer atmospheres - of the Sun and more distant stars. Astronomers have learned that the multi-million degree atmospheres of the Sun and solar-like stars are highly structured. They are dominated by a constantly moving magnetic field which is generated inside the star.

The atmospheric plasma, made up of electrically charged atoms of gas and free electrons, is bound and shaped by the magnetic field lines. These form a continuous pattern of magnetic loops across the surface of the star.

Resembling invisible elastic bands, these magnetic field lines can be stretched and altered by sudden disturbances, such as the blast of a shock wave from a flare. A sequence of waves is then created in the loops. The period and amplitude of these standing waves depends on the local magnetic field strength, the plasma density and temperature, and the length of the loop.

The study of coronal structures with the help of such oscillations is called "coronal seismology", a technique comparable to geoseismology, which studies the internal structure of the Earth through motions of earthquake waves.

On the Sun, these looping structures can be observed directly with high-resolution space telescopes, such as the Solar and Heliospheric Observatory (SOHO) and the Transition Region and Coronal Explorer (TRACE).

However, observations on more distant stars are quite rare. The principal reason is that the stars are too far away for their disks to be spatially resolved. Any flares and variations in brightness must be strong enough to be observed above the overall light output from a star.

Speaking today at the RAS UK Solar Physics Meeting in Birmingham, Urmila Mitra-Kraev (Mullard Space Science Laboratory - UCL) explained how her team detected the first X-ray fluctuations seen in a flare that exploded on a distant star, and the significance of this detection.

The flare, which occurred on the active, M-type dwarf star known as AT Microscopii (AT Mic), was observed with the European Space Agency's orbiting XMM-Newton X-ray observatory on 16 October 2000. This cool star, about half the diameter of the Sun, is located 33 light years away from Earth in the southern sky constellation of Microscopium.

Studies of the star at X-ray wavelengths were obtained over a period of almost seven hours, using the European Photon Imaging Camera (EPIC). While the spacecraft was staring at the star, there was a rapid increase in X-ray emissions as a large flare erupted. The flare lasted for a period of 1 hour 25 minutes and reached a temperature of about 24 million degrees Celsius.

"Since stars are only seen as point-like sources, a stellar oscillation can only be observed if it is strong enough to be above the full-disk background emission," said Mitra-Kraev. "In this case, however, it provided a great opportunity to estimate the length of the oscillating loop and the local magnetic field strength, both of which are not directly observable."

The oscillation was seen in the soft X-ray band, while the density and temperature can be obtained from the medium and high-resolution X-ray spectra. Based on previous studies of wave motions on the Sun, the team was able to estimate the size and magnetic strength of the loop associated with the stellar flare.

"The strength of the local magnetic field was about 100 Gauss - a value in the upper range for solar flares," said Mitra-Kraev. "This oscillation was a rare opportunity to quantitatively measure the coronal magnetic field strength of a magnetically highly active star.

"We also determined that the loop arched across AT Mic for a distance of 250,000 km - equivalent to stretching over 20 Earths strung out side by side. Although it sounds large, this is actually comparable to the sizes of large flare structures on the Sun and other stellar loops that have been measured by different methods.

"Using our knowledge of coronal seismology on the Sun and applying it to more distant stars is a promising new way to probe stellar atmospheres," she added. "It will also enable us to measure properties such as the lengths of loops linked with stellar flares and the strengths of coronal magnetic fields on stars that are too small to be studied with ordinary telescopes."

Royal Astronomical Society (RAS)