The gas content of discs in transition
          William R.F. Dent, UKATC

          The discs found around recently formed stars have a gas:dust ratio close to that of their parent molecular clouds. By contrast debris discs around older main sequence stars have little or no detectable gas. We describe results from a study of submm CO lines in discs around F-B stars over a range of ages. The results show a significant gas mass can exist in discs of age up to ~20Myr. As long as the dust density is sufficient to shield the gas from photodissociation, CO is inevitably found in these circumstellar discs.  The size of the gas disc appears to be independent of the stellar spectral type.  However, the apparent gas disc radius decreases with time, with typical sizes of ~200au for disc ages of 3-7Myr, and ~70au for 7-20Myr. These extended discs will provide a gas reservoir of > 1 Earth mass of H2 in the outer pre-Kuiper Belt regions for a period of ~20Myr.