  Observations of submillimeter lines of CO (up to J=6-5), HCO+, HCN (up to
J=4-3) and their isotopes from circumstellar disks around low mass pre-main
sequence stars are presented. The different line ratios and optical depths
indicate that most of the observed line emission arises from an intermediate
disk layer with high densities of 1E6-1E8 cm-3 and moderately warm temperatures
in the outer regions. The data are compared with three disk models from the
literature using a full 2D Monte Carlo radiative transfer code. The abundances
of the molecules are constrained from the more optically thin 13C species and
indicate depletions of 1-30 for LkCa 15 and very high depletions of > 100 for
TW Hya with respect to dark cloud abundances. Evidence for significant
freeze-out (factors 10 or larger) of CO and HCO+ onto grain surfaces at
temperatures below 22 K is found, but the abundances of these molecules must
also be low in the warmer upper layer, most likely as a result of
photodissociation. A warm upper layer near the surface of a flaring disk heated
by stellar and interstellar radiation is an appropriate description of the
observations of TW Hya. LkCa 15 seems to be cooler at the surface, perhaps due
to dust settling.
