Weekly Science Update

Friday, December 26, 2008
X-Ray Emission from Young Stars
A newly-formed star usually has a disk of gas and dust around it. This disk (the source of possible future planets) can generate intense X-ray radiation as its material falls onto the star's surface. Indeed, X-ray emission from such stars has been seen and studied by the Chandra X-ray Observatory and other X-ray facilities. Besides being a powerful diagnostic of what is going on around the infant star, the radiation affects the chemistry in the disk, and can drive a hot wind that influences the morphology of the disk and its developing planets.

SAO astronomers Barbara Ercolano, Jeremy Drake, John Raymond, and a colleague of theirs have completed the first stage of a detailed computational code that models the hot gas around young stars. The sophisticated computation finds the X-ray emitting gas can reach temperatures of about one million kelvin near the star, and ten thousand kelvin at the distance of one AU (where a putative earth might be). They find that several atomic species are excited in these regions and should be observable with new telescopes, and they predict the intensities so that eventual comparisons can be used to refine their model. They also are able to predict mass-loss rates: about one-thousandth of an earth-mass per year, enough to disburse the disk in a few million years if the wind were to continue unabated.

The calculations were particularly complex because a wide range of temperatures had to be included in the model, from extremely hot to more normal values of a few thousand kelvin; the computations included effects of ultraviolet as well as X-ray radiation. Complicating the calculation, the disk itself rotates and evolves, and has a molecular component mixed in with the dust that must be considered. The team's result, a detailed characterization of the corona-like inner region of the hot disk around young stars, represents an important step in unraveling both the evolving structure of young disks, and how the hot inner regions of a young star affect the solar system under development.

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