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We carried out a 12CO(3-2) survey of 52 southern stars with a wide range ofIR excesses (LIR/L*) using the single dish telescopes APEX and ASTE. The mainaims were (1) to characterize the evolution of molecular gas in circumstellardisks using LIR/L* values as a proxy of disk dust evolution, and (2) toidentify new gas-rich disk systems suitable for detailed study with ALMA. About60% of the sample (31 systems) have LIR/L* > 0.01 typical of T-Tauri or HerbigAeBe stars, and the rest (21 systems) have LIR/L* < 0.01 typical of debrisdisks. We detect CO(3-2) emission from 20 systems, and 18 (90%) of these haveLIR/L* > 0.01. However, the spectra of only four of the newly detected systemsappear free of contamination from background or foreground emission frommolecular clouds. These include the early-type stars HD 104237 (A4/5V, 116 pc)and HD 98922 (A2 III, 507 pc, as determined in this work), where ourobservations reveal the presence of CO-rich circumstellar disks for the firsttime. Of the other detected sources, many could harbor gaseous circumstellardisks, but our data are inconclusive. For these two newly discovered gas-richdisks, we present radiative transfer models that simultaneously reproduce theirspectral energy distributions and the 12CO(3-2) line profiles. For both ofthese systems, the data are fit well by geometrically flat disks, placing themin the small class of non-flaring disks with significant molecular gasreservoirs.

A CO survey in planet-forming disks: characterizing the gas content in the epoch of planet formation