Protoplanetary disc evolution and dispersal: the implications of X-ray photoevaporation

We explore the role of X-ray photoevaporation in the evolution and dispersal of viscously evolving T Tauri discs. We show that the X-ray photoevaporation wind rates scale linearly with X-ray luminosity, such that the observed range of X-ray luminosities for solar-type T Tauri stars (1028-1031 erg s-1) gives rise to vigorous disc winds with rates of the order of 10-10 to 10-7 M-circle dot yr-1. These mass-loss rates are comparable to typically observed T Tauri accretion rates, immediately demonstrating the relevance of X-ray photoevaporation to disc evolution. We use the wind solutions from radiation-hydrodynamic models, coupled to a viscous evolution model, to construct a population synthesis model so that we may study the physical properties of evolving discs and so-called 'transition discs'. Current observations of disc lifetimes and accretion rates can be matched by our model assuming a viscosity parameter alpha = 2.5 x 10-3. Our models confirm that X-rays play a dominant role in the evolution and dispersal of protoplanetary discs giving rise to the observed diverse population of inner-hole 'transition' sources which include those with massive outer discs, those with gas in their inner holes and those with detectable accretion signatures. To help understand the nature of observed transition discs we present a diagnostic diagram based on accretion rates versus inner-hole sizes that demonstrate that, contrary to recent claims, many of the observed accreting and non-accreting transition discs can easily be explained by X-ray photoevaporation. However, we draw attention to a smaller but still significant population of strongly accreting (similar to 10-8 M-circle dot yr-1) transition discs with large inner holes (> 20 au) that lie outside the predicted X-ray photoevaporation region, suggesting a different origin for their inner holes. Finally, we confirm the conjecture of Drake et al. that accretion is suppressed by the X-rays through 'photoevaporation-starved accretion' and predict that this effect can give rise to a negative correlation between X-ray luminosity and accretion rate, as reported in the Orion data. We also demonstrate that our model can replicate the observed difference in X-ray properties between accreting and non-accreting T Tauri stars.