Abstract

I first recapitulate former work explaining how the interplay between viscous evolution and extreme ultraviolet (EUV) photoevaporation produces a characteristic pattern of disc clearing, which invoves first rapid viscous draining of the inner disc (within a few A.U.) followed by rapid photoevaporation of the outer disc. This behaviour sets in at late times when the accretion rate through the disc is very low ($ im 10 ^{M_odot$ yr$}{-1}$).

I then describe recent work which demonstrates that, contrary to previous estimates, Xray photoevaporation is in fact likely to be a major disc dispersal agent. The sequence of disc clearing phases is qualitatively similar to that described above but with two key differences: i) the photoevaporation rate is ten times higher and thus this clearing sets in earlier, when the disc accretion rate is $ im 10 ^{M_odot$ yr$}{-1}$ and ii) a combination of the greater penetrating power of Xrays and the somewhat lower temperatures attained by Xray heated gas compared with the EUV case means that the peak wind mass loss occurs at $ im 20 $ A.U.. The size of the inner hole is thus $ im 4$ times larger than in EUV photoevaporative models.

We discuss the implications of this new result for models of disc clearing and the production of transition discs.