Coordinated effort of space observatories detects new type of stellar flare

June 04, 2000

A group of astronomers using the Hubble Space Telescope and Chandra X-ray Observatory, in concert with other telescopes, have directly detected for the first time a new type of stellar flare occurring in a narrow temperature range of gas on a star other than the sun.

Rachel Osten, a graduate student at the University of Colorado at Boulder, said the flares were detected on a star, known as binary system HR 1099, in a very narrow temperature range of gas recorded in the ultraviolet portion of the light spectrum, but not in the higher energy X-ray region.

"These flares provide a direct detection on a star of a phenomenon seen previously only on the sun," Osten said. "The latest results will lead to improved knowledge of flare physics by increasing our knowledge of the types of activities that occur on other stars, and challenging current models that attempt to explain flares."

Osten, who is affiliated with CU-Boulder's Center for Astrophysics and Space Astronomy, presented the findings at the national meeting of the American Astronomical Society June 5 in Rochester, New York. Thomas Ayres, Alexander Brown and Jeffrey Linsky, all of CU-Boulder, worked on the project.

Stellar flares are violent releases of energy and gas from the outer atmospheres of stars. The sun and many sun-like stars produce flares, and binary stars like HR 1099 can experience flares that are a million times more energetic than the largest flare seen on the sun, she said.

Gaining a better understanding of how flares happen on other stars will help scientists figure out what causes them on the sun, where they cause geomagnetic storms that can trigger power outages and communication blackouts on Earth.

"Since flares come in many shapes and sizes, and because stars like HR 1099 emit light across the spectrum from radio waves to X-rays, they must be examined in a variety of wavelength ranges at the same time to determine the conditions present on the star," Osten said, noting the importance of coordinating the different telescopes.

The flares on HR 1099 were only seen at 100,000 degrees, recorded by spectral lines in the ultraviolet range accessible to the Hubble, while spectra taken by Chandra in the X-ray region, which probe temperatures near one million degrees, showed no brightening of lines. The ultraviolet lines broadened dramatically, according to Osten, and reached supersonic velocities during the several hours that the flares occurred.

Osten said this is the first time something like this has been observed in concert with a high resolution X-ray telescope to prove conclusively that these flares only occur in relatively warm gas, and not the hot gas that emits X-rays.

Previous work by Thomas Ayres, Brian Wood and Jeffrey Linsky of CU-Boulder using ultraviolet spectral studies suggested that this phenomenon could be present in sun-like stars.

"These observations demonstrate the unique opportunity for simultaneous ultraviolet and X-ray spectroscopic observations with the Hubble Space Telescope and Chandra that is now available to astronomers," said Alexander Brown, of CU-Boulder's Center for Astrophysics and Space Astronomy, who worked with Osten on the research.

In September 1999, the Extreme Ultraviolet Explorer satellite focused on binary system HR 1099, 90 light-years from Earth, for nine days registering only small variations. During that time, the Chandra X-ray Observatory focused its spectrometer for a day and a half cataloguing light changes, while the National Science Foundation's Very Large Array observed radio emission changes and the Hubble Space Telescope chronicled the ultraviolet spectrum changes for seven hours.
A paper has been submitted for publication to the Astrophysical Journal and includes the work of Thomas Ayres, Alexander Brown, Rachel Osten and Jeffrey Linsky of CU-Boulder, as well as David Huenemoerder of MIT and Jeremy Drake and Nancy Brickhouse of the Harvard-Smithsonian Center for Astrophysics.

NASA's Office of Space Science and NASA's Graduate Student Research Program supported the work.

To view an image similar to a stellar flare on star HR 1099, visit the Web site at

University of Colorado at Boulder

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