Abstract

We carried out a (CO)-C-12(3-2) survey of 52 southern stars with a wide range of IR excesses (L-IR/L-*) using the single-dish telescopes APEX and ASTE. The main aims were (1) to characterize the evolution of molecular gas in circumstellar disks using L-IR/L-* values as a proxy of disk dust evolution, and (2) to identify new gas-rich disk systems suitable for detailed study with ALMA. About 60% of the sample (31 systems) have L-IR/L-* > 0.01, typical of T Tauri or Herbig AeBe stars, and the rest (21 systems) have L-IR/L-* <0.01, typical of debris disks. We detect CO(3-2) emission from 20 systems, and 18 (90%) of these have L-IR/L-* > 0.01. However, the spectra of only four of the newly detected systems appear free of contamination from background or foreground emission from molecular 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 our observations reveal the presence of CO-rich circumstellar disks for the first time. Of the other detected sources, many could harbor gaseous circumstellar disks, but our data are inconclusive. For these two newly discovered gas-rich disks, we present radiative transfer models that simultaneously reproduce their spectral energy distributions and the (CO)-C-12(3-2) line profiles. For both of these systems, the data are fit well by geometrically fiat disks, placing them in the small class of non-flaring disks with significant molecular gas reservoirs.