Gas evolution in protoplanetary disks

We present the results of a study of the evolution of the gas and dust in disks around T Tauri, Herbig Ae and Vega-like stars. We observed the two lowest rotational lines of H-2 with the ISO-SWS as well as (CO)-C-12 3-2 and (CO)-C-13 3-2 with the JCMT, and CO 6-5 with the CSO. The H-2 lines trace the warm (similar to 100 K) inner region whereas the CO lines probe the celder outer disks. Substantial amounts of H-2 have been detected toward T Tauri and Herbig Ae stars, and, surprisingly, also around three Vega-like objects (49 Cet, HD 135344 and beta Pictoris). In contrast with previous conclusions derived from CO data, a significant mass of warm gas is found to persist up to ages of several tens of Myrs, suggesting that slow formation of gas-rich giant planets is possible.