Fluxes and upper limits (at 3σ confidence) from our APEX observations of the [C i] 1−0 and 2–1 and the CO 6−5 transitions. Large circles indicate sources with a detection on at least one axis. Empty symbols mark sources for which only the [C i] 1−0 line was observed. CO 6−5 and [C i] 2−1 were always observed in parallel. Colours show the stellar effective temperature: late-type stars are red and early-type stars blue. The black line connects fiducial disk models from Sect. 4 for the stellar spectral type range of our observations (diamonds from red to blue, T_eff∈ [4000,12 000] K in steps of 2000 K, assumed distance 140 pc).