Molecular gas in irradiated protoplanetary discs

Gas discs around young stars typically disappear after a few Myr and it has been suggested that the discs are internally photoevaporated, but the stellar ultraviolet and X-ray radiation appears insufficient for this mechanism to work. Archival data are used to search for any relation between X-ray luminosity and gas in the disc (traced by (CO)-C-13 molecules) for isolated young stars in Taurus. A significant correlation is found in that more irradiation increases the amount of (CO)-C-13, rather than dispersing the disc. Since high dust masses and accretion rates do not boost the X-ray luminosities, feeding of the radiation by the disc can be ruled out. It is more plausible that energetic photons are releasing molecules that would otherwise be frozen onto dust grains. In this case, estimates of disc mass using a canonical CO:H-2 ratio are unrealistic. However, the mass released in CO molecules could contribute to building a belt of icy comets similar to the Sun's Kuiper Belt.