Diagram showing the physical parameters which dominate the emission of the CO gas and dust (see Sect. 4.2 for details). The higher part of the plot represents the optically thick regime of the gas, and the lower part represents the optically thin regime. In a hybrid disk such as HD 141569, the dust may evolve faster than the gas in a first step, moving the disk out of the correlation. In a second step, the natural gas dissipation moves downwards in position. On the contrary, debris disks are expected to lie in the optically thin region. For β Pic, the position of the disk in the diagram could be higher than expected because of the gas enhancement produced by collisional events. For both sources, the arrows show the expected path in the diagram resulting from their possible evolutions.