The origin of gas in the two oldest comma newly discovered gaseous debris disks.

Many young mainsequence stars are surrounded by dusty debris disks, but only very few of them have a detectable gas component. In our APEX survey we discovered two new debris disks containing a substantial amount of molecular CO gas. One of them, the 30 millionyearold HD 21997, is the oldest known gaseous debris disk, making it the bestcandidate for containing CO gas of secondary origin, produced by sublimation of planetesimals, photodesorption from dust grains, or vaporization of colliding dust particles.We suggest that our discoveries together with the already known objects beta Pic and 49 Ceti form a distinguished group of debris systems that may represent the first stage of gas evolution after the primordial phase.Here we propose new Herschel observations to deduce the physical properties and the origin of gasin our targets, and study the possible physical processes of gas production in the secondary origin scenario.Our immediate objectives are the following: 1) Detection of O I 63um and C II 158um lines in our targets; 2) physical characterization of the gas/dust disk components; 3) establish the origin of gas and 4) comparison with beta Pic and 49 Ceti, and studying evolutionary aspects.The observation of atomic emission lines in such disks will provide a valuable information on thecomposition of the released gas and thus on similarity of the volatile composition of SolarSystem comets to exosolar planetesimals, potentially playing key role in the delivery of volatiles to exoplanets. The study request 9.2h observing time. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]