  We studied several representative circumstellar disks surrounding the Herbig
Ae star MWC 480 and the T Tauri stars LkCa 15 and DM Tau at (sub-)millimeter
wavelengths in lines of CCH. Our aim is to characterize photochemistry in the
heavily UV-irradiated MWC 480 disk and compare the results to the disks around
cooler T Tauri stars. We detected and mapped CCH in these disks with the IRAM
Plateau de Bure Interferome- ter in the C- and D-configurations in the (1-0)
and (2-1) transitions. Using an iterative minimization technique, the CCH
column densities and excitation conditions are con- strained. Very low
excitation temperatures are derived for the T Tauri stars. These values are
compared with the results of advanced chemical modeling, which is based on a
steady-state flared disk structure with a vertical temperature gradient, and a
gas- grain chemical network with surface reactions. Both model and observations
suggest that CCH is a sensitive tracer of the X-ray and UV irradiation. The
predicted radial dependency and source to source variations of CCH column
densities qualitatively agree with the observed values, but the predicted
column densities are too low by a factor of several. The chemical model fails
to reproduce high concentrations of CCH in very cold disk midplane as derived
from the observed low excitation condition for both the (1-0) and (2-1)
transitions.
