  We have investigated the two-dimensional (R,Z) distribution of deuterated
molecular species in circumstellar disks around young stellar objects. The
abundance ratios between singly deuterated and normal molecules (``D/H
ratios'') in disks evolve in a similar way as in molecular clouds.
Fractionation is caused by rapid exchange reactions that are exothermic because
of energy differences between deuterated and normal species. In the midplane
region, where molecules are heavily depleted onto grain surfaces, the D/H
ratios of gaseous molecules are higher than at larger heights. The D/H ratios
for the vertical column densities of NH3, H2O, and HCO+ are sensitive to the
temperature, and decrease significantly with decreasing radial distance for R <
300 AU. The analogous D/H ratios for CH4 and H2CO, on the other hand, are not
very sensitive to the temperature in the range (T=10-50 K) we are concerned
with, and do not decrease with decreasing R at R > 50 AU. The D/H
column-density ratios also depend on disk mass. In a disk with a larger mass,
the ratios of deuterated species to normal species are higher, because of
heavier depletion of molecules onto grains. In the second part of the paper, we
report molecular column densities for disks embedded in ambient cloud gas. Our
results suggest that CN and HCO+ can be tracers of gaseous disks, especially if
the central object is a strong X-ray source. Our results also suggest that the
radial distributions of CN, C2H, HCN, and H2CO may vary among disks depending
on the X-ray luminosity of the central star.
