X-ray radiative transfer in protoplanetary disks - The role of dust and
  X-ray background fields
  The X-ray luminosities of T Tauri stars are about two to four orders of
magnitude higher than the luminosity of the contemporary Sun. As these stars
are born in clusters, their disks are not only irradiated by their parent star
but also by an X-ray background field produced by the cluster members. We aim
to quantify the impact of X-ray background fields produced by young embedded
clusters on the chemical structure of disks. Further, we want to investigate
the importance of the dust for X-ray radiative transfer in disks.
  We present a new X-ray radiative transfer module for the radiation
thermo-chemical disk code ProDiMo, which includes X-ray scattering and
absorption by both the gas and dust component. For the X-ray radiative
transfer, we consider irradiation by the star and by X-ray background fields.
To study the impact of X-rays on the chemical structure of disks we use the
well-established disk ionization tracers N2H+ and HCO+.
  For evolved dust populations, X-ray opacities are mostly dominated by the
gas; only for photon energies $E\gtrsim5-10\,$keV, dust opacities become
relevant. Consequently, the local disk X-ray radiation field is only affected
in dense regions close to the disk midplane. X-ray background fields can
dominate the local X-ray disk ionization rate for disk radii $r\gtrsim20\,$au.
However, the N2H+ and HCO+ column densities are only significantly affected in
case of low cosmic-ray ionization rates, or if the background flux is at least
a factor of ten higher than the flux level expected for clusters typical for
the solar vicinity. Observable signatures of X-ray background fields in
low-mass star-formation regions, like Taurus, are only expected for cluster
members experiencing a strong X-ray background field. For the majority of the
cluster members, the X-ray background field has only little impact on the disk
chemical structure.
