  The overall properties of disks surrounding intermediate PMS stars (HAe) are
not yet well constrained by current observations. The disk inclination, which
significantly affect SED modeling, is often unknown. We attempted to resolve
the disks around CQ Tau and MWC 758, to provide accurate constraints on the
disk parameters, in particular the temperature and surface density
distribution. We report arcsecond resolution observations of dust and CO line
emissions with the IRAM array. The disk properties are derived using a standard
disk model. We use the Meudon PDR code to study the chemistry. The two disks
share some common properties. The mean CO abundance is low despite disk
temperatures above the CO condensation temperature. Furthermore, the CO surface
density and dust opacity have different radial dependence. The CQ Tau disk
appears warmer, and perhaps less dense than that of MWC 758. Modeling the
chemistry, we find that photodissociation of CO is a viable mechanism to
explain the low abundance. The photospheric flux is not sufficient for this: a
strong UV excess is required. In CQ Tau, the high temperature is consistent
with expectation for a PDR. The PDR model has difficulty explaining the mild
temperatures obtained in MWC 758, for which a low gas-to-dust ratio is
preferred. A yet unexplored alternative could be that, despite currently high
gas temperatures, CO remains trapped in grains, as the models suggest that
large grains can be cold enough to prevent thermal desorption of CO. The low
inclination of the CQ Tau disk, ~30^\circ, challenges previous interpretations
given for the UX Ori - like luminosity variations of this star. We conclude
that CO cannot be used as a simple tracer of gas-to-dust ratio, the CO
abundance being affected by photodissociation, and grain growth.
