Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21710402g&link_type=abstract
American Astronomical Society, AAS Meeting #217, #104.02; Bulletin of the American Astronomical Society, Vol. 43, 2011
Astronomy and Astrophysics
Astronomy
Scientific paper
Young stellar objects radiate large numbers of keV-photons that are able to penetrate significant columns of gas and thus ionize and heat circumstellar material, including the accretion disks and outflows of very young stars. The unshielded ionization rates are high. At a distance of 1 AU from a young stellar object with a moderately strong X-ray source, they can be 100 million times the standard ionization rate associated with galactic cosmic rays. X-ray irradiation of a typical T Tauri star disk generates a layered atmosphere, ranging from a thin H II region through hot and warm regions on top of the main accretion disk. In the transition from warm to cool, the gas changes from atomic to molecular. This transition region produces a rich set of emission lines from the ultraviolet through the infrared. A good diagnostic of the effects of X-rays from young stellar objects is the Ne II fine structure line at 12.8 microns. This line has now been detected in more than fifty young stellar objects by Spitzer and ground-based telescopes. Since X-rays from young stellar objects penetrate disks down to vertical column densities of order 10 g per square cm, they help determine the location of the active accretion layer of disks governed by the magneto-centrifugal instability. In addition to directly probing the disk, stellar X-rays affect the stellar and disk winds of young stellar objects, including photo-evaporative flows, and thereby influence the dispersal of circumstellar disks.
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