Weekly Science Update

Wednesday, January 16, 2008
X-rays from Orion
Stars emit X-rays. Our sun, for example, has a hot corona entwined with magnetic fields, and emits X-rays. Astronomers have discovered that young stars emit considerably more X-rays than older, sun-like stars, but it is not clear why, nor when in the life of a new star the X-ray emission begins, nor how the emission subsequently changes. For example, it has been established that those young stars with accretion and winds emit X-rays -- perhaps the interaction between the ionized outflows and magnetic fields in the disks contribute to the strong X-rays.

SAO astronomer Scott Wolk was part of a team of twelve astronomers that combined data from the Chandra X-ray Observatory and four other observatories using four wavelength regions to investigate the mystery of X-ray emission from young stars. They chose the rich, young stellar cluster in the Orion nebula, a relatively nearby site of star formation that includes members in a wide range of evolutionary stages, including forty-five very young stars that are probably less than a million years old. The scientists used their optical and infrared data to more accurately determine the ages and properties of the candidates, and the Chandra data to characterize the X-rays. This is the first such research effort with a statistically adequate sample to determine the X-ray properties of young stars.

The astronomers conclude that X-rays begin at a very early stage in the life of a star; depending on the star's mass, this can be less than one hundred thousand years. The X-ray emission then increases with age, even beyond the age at which the star's accretion is thought to cease. The results imply that there is only one mechanism that produces moderate-strength X-rays in these young stars, and that this mechanism arises from stellar magnetic activity and does not depend on either accretion or circumstellar disks.

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