  We study the molecular content and chemistry of a circumstellar disk
surrounding the Herbig Ae star AB Aur at (sub-)millimeter wavelengths. Our aim
is to reconstruct the chemical history and composition of the AB Aur disk and
to compare it with disks around low-mass, cooler T Tauri stars. We observe the
AB Aur disk with the IRAM Plateau de Bure Interferometer in the C- and D-
configurations in rotational lines of CS, HCN, C2H, CH3OH, HCO+, and CO
isotopes. Using an iterative minimization technique, observed columns densities
and abundances are derived. These values are further compared with results of
an advanced chemical model that is based on a steady-state flared disk
structure with a vertical temperature gradient, and gas-grain chemical network
with surface reactions. We firmly detect HCO+ in the 1--0 transition,
tentatively detect HCN, and do not detect CS, C2H, and CH3OH. The observed HCO+
and 13CO column densities as well as the upper limits to the column densities
of HCN, CS, C2H, and CH3OH are in good agreement with modeling results and
those from previous studies. The AB Aur disk possesses more CO, but is less
abundant in other molecular species compared to the DM Tau disk. This is
primarily caused by intense UV irradiation from the central Herbig A0 star,
which results in a hotter disk where CO freeze out does not occur and thus
surface formation of complex CO-bearing molecules might be inhibited.
