Chemical Processes in Protoplanetary Disks II. On the importance of
  photochemistry and X-ray ionization
  We investigate the impact of photochemistry and X-ray ionization on the
molecular composition of, and ionization fraction in, a protoplanetary disk
surrounding a typical T Tauri star. We use a sophisticated physical model,
which includes a robust treatment of the radiative transfer of UV and X-ray
radiation, and calculate the time-dependent chemical structure using a
comprehensive chemical network. In previous work, we approximated the
photochemistry and X-ray ionization, here, we recalculate the photoreaction
rates using the explicit UV wavelength spectrum and wavelength-dependent
reaction cross sections. We recalculate the X-ray ionization rate using our
explicit elemental composition and X-ray energy spectrum. We find
photochemistry has a larger influence on the molecular composition than X-ray
ionization. Observable molecules sensitive to the photorates include OH, HCO+,
N2H+, H2O, CO2 and CH3OH. The only molecule significantly affected by the X-ray
ionization is N2H+ indicating it is safe to adopt existing approximations of
the X-ray ionization rate in typical T Tauri star-disk systems. The
recalculation of the photorates increases the abundances of neutral molecules
in the outer disk, highlighting the importance of taking into account the shape
of the UV spectrum in protoplanetary disks. A recalculation of the
photoreaction rates also affects the gas-phase chemistry due to the adjustment
of the H/H2 and C+/C ratios. The disk ionization fraction is not significantly
affected by the methods adopted to calculate the photochemistry and X-ray
ionization. We determine there is a probable 'dead zone' where accretion is
suppressed, present in a layer, Z/R <~ 0.1 - 0.2, in the disk midplane, within
R \approx 200 AU.
