  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. We used the EMIR
receivers recently installed at the IRAM 30m telescope to carry a sensitive
search for molecular lines in the disks surrounding AB Aur, DM Tau, and LkCa
15. 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 the
molecular species observed so far, SO is not as abundant in DM Tau as it is in
AB Aur. 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.
