  Aims. Our goal is to determine the molecular composition of the circumstellar
disk around AB Aurigae (hereafter, AB Aur). AB Aur is a prototypical Herbig Ae
star and the understanding of its disk chemistry is of paramount importance to
understand the chemical evolution of the gas in warm disks. Methods. We used
the IRAM 30-m telescope to perform a sensitive search for molecular lines in AB
Aur as part of the IRAM Large program ASAI (A Chemical Survey of Sun-like
Star-forming Regions). These data were complemented with interferometric
observations of the HCO+ 1-0 and C17O 1-0 lines using the IRAM Plateau de Bure
Interferometer (PdBI). Single-dish and interferometric data were used to
constrain chemical models. Results. Throughout the survey, several lines of CO
and its isotopologues, HCO+, H2CO, HCN, CN and CS, were detected. In addition,
we detected the SO 54-33 and 56-45 lines, confirming the previous tentative
detection. Comparing to other T Tauri's and Herbig Ae disks, AB Aur presents
low HCN 3-2/HCO+ 3-2 and CN 2-1/HCN 3-2 line intensity ratios, similar to other
transition disks. AB Aur is the only protoplanetary disk detected in SO thus
far. Conclusions. We modeled the line profiles using a chemical model and a
radiative transfer 3D code. Our model assumes a flared disk in hydrostatic
equilibrium. The best agreement with observations was obtained for a disk with
a mass of 0.01 Msun , Rin=110 AU, Rout=550 AU, a surface density radial index
of 1.5 and an inclination of 27 deg. The intensities and line profiles were
reproduced within a factor of 2 for most lines. This agreement is reasonable
taking into account the simplicity of our model that neglects any structure
within the disk. However, the HCN 3-2 and CN 2-1 line intensities were
predicted more intense by a factor of >10. We discuss several scenarios to
explain this discrepancy.
