X‐Ray Ionization of Protoplanetary Disks

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Abstract
In the light of new observations of star forming regions by the Advanced Satellite for Cosmology and Astrophysics (ASCA) and by ROSAT, we assess the ability of young stars to ionize their own circumstellar disks with stellar coronal X-rays. Although stellar winds may absorb soft X-rays, hard X-rays can penetrate to large column densities and, until they are absorbed, produce ionization rates greater than standard estimates for Galactic cosmic rays. As in previous studies of the external ionization of protoplanetary disks by cosmic rays, we find that X-ray ionization produces a surface layer that is well coupled to disk magnetic fields at ~AU distances. The properties of the surface layer depend on the characteristics of the X-ray source and, thus, on the evolutionary status of the central star. Even if Galactic cosmic rays are efficiently excluded by magnetized winds, stellar X-ray irradiation alone may provide sufficient ionization for disks to accrete via the Balbus-Hawley instability. The resulting vertically layered structure of disks at ~AU distances (a well-coupled surface layer overlying a poorly coupled, deeper layer) may lead to divergent dynamical evolution in the two regions. While accretion in the surface layer contributes to the buildup of the mass of the star, the quiescent conditions in the poorly coupled, deeper layer appear to be conducive to the formation of planets.