Contribution Invited Talk
Origin, evolution and frequency of gas in debris disks
A growing number of main sequence stars show evidence for circumstellar gas. It is most often CO that is detected, which is puzzling because this molecule should have a short lifetime due to photo-dissocation by the interstellar radiation field. Since this gas is usually found around young stars, the most obvious explanation would be that this is a remnant of the primordial protoplanetary disk that provides key clues about disk dispersal processes. However, in the case of beta Pictoris there is strong evidence from the disk morphology that the gas is secondary, i.e., produced in the destruction of icy planetesimals. To interpret these observations, and to model how the gas evolves, it has been necessary to develop new models, because the low optical depth of debris disk environments and non-standard gas composition invalidates the assumptions used in protoplanetary disk modelling. This talk will review our understanding of gas in debris disks, starting with the frequency with which main sequence stars show evidence for gas. We will present new ALMA observations of CO in the beta Pic disk that map its distribution and provide valuable information on the physical conditions in this disk. We will also discuss a new model for the origin and evolution of gas in debris disks that includes the evolution of the photo-dissociation products that can be constrained observationally.