Chemical composition of the circumstellar disk around AB Aurigae

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 paramount for understanding 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 previously tentative detection. Compared to other T Tauri 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, and its detection is consistent with interpretation of this disk being younger than those associated with T Tauri stars. 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 M_⊙, R_in = 110 AU, R_out = 550 AU, a surface density radial index of 1.5, and an inclination of 27°. The intensities and line profiles were reproduced within a factor of ∼2 for most lines. This agreement is reasonable considering 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 to be more intense by a factor of >10. We discuss several scenarios to explain this discrepancy.