Aims. Very few molecular species have been detected in circumstellar disks surrounding young stellar objects. We are carrying out an observational study of the chemistry of circumstellar disks surrounding T Tauri and Herbig Ae stars. First results of this study are presented in this note. Methods. We used the EMIR receivers recently installed at the IRAM?30?m telescope to carry out a sensitive search for molecular lines in the disks surrounding AB?Aur, DM?Tau, and LkCa?15. Results. We detected lines of the molecules HCO+, CN, H2CO, SO, CS, and HCN toward AB Aur. In addition, we tentatively detected DCO+ and H2S lines. The line profiles suggest that the CN, HCN, H2CO, CS, and SO lines arise in the disk. This makes it the first detection of SO in a circumstellar disk. We have unsuccessfully searched for SO toward DM Tau and LkCa?15, and for c-C3H2 toward AB?Aur, DM?Tau, and LkCa?15. Our upper limits show that contrary to all molecular species observed so far, SO is not as abundant in DM?Tau as it is in AB?Aur. Conclusions. Our results demonstrate that the disk associated with AB Aur is rich in molecular species. Our chemical model shows that the detection of SO is consistent with that expected from a very young disk where the molecular adsorption onto grains does not yet dominate the chemistry.
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