X-ray Ionization of Heavy Elements Applied to Protoplanetary Disks
  The consequences of the Auger effect on the population of heavy element ions
are analyzed for the case of relatively cool gas irradiated by keV X-rays, with
intended applications to the accretion disks of young stellar ob jects. Highly
charged ions are rapidly reduced to the doubly-charged state in neutral gas, so
the aim here is to derive the production rates for these singly- and
doubly-charged ions and to specify their transformation by recombination,
charge transfer, and molecular reactions. The theory is illustrated by
calculations of the abundances of eleven of the most cosmically abundant heavy
elements in a model of a typical T Tauri star disk. The physical properties of
the gas are determined with an X-ray irradiated thermal-chemical model, which
shows that the disk atmosphere consists of a hot atmosphere overlaying the
mainly cool body of the disk. There is a warm transition layer where hydrogen,
carbon and oxygen are transformed from atomic to molecular species and the
ionization drops by several orders of magnitude. The doubly charged ions are
then largely confined to the hot outer layers of the disk. The nature of the
dominant ions below the transition depends sensitively on the poorly
constrained abundances of the heavy elements. Observational consequences and
connections with the active layers of the magneto-rotational instability are
briefly discussed.
