Dispersal of protoplanetary disks: Effects of photoevaporation with stellar evolution and MHD winds

We investigate the long-term evolution and dispersal of protoplanetary disks. We specifically focus on the following two questions: (i) How does stellar evolution affect disk evolution? (ii) How do disks with weak turbulence disperse? On the first question, since photoevaporation (PE) is driven by stellar high-energy photons, we first derived the evolution of stellar XUV luminosities by combining stellar evolution simulations, stellar atmospheric models, and empirical relations from observations and theoretical considerations. From disk evolution simulations including a time-dependent PE model, we found that PE rates around low-mass stars are almost constant with time. On the other hand, those around intermediate-mass stars change dramatically: the X-ray PE rate decreases with time due to the stellar structure evolution (i.e., convective to radiative), whereas the FUV increases due to the increase of the stellar effective temperature. We conclude that stellar evolution is crucially important for the disk evolution around intermediate-mass stars. Our results show that the disk lifetime decreases with stellar mass, which has been suggested by observations. On the second question, both recent observations and theoretical studies have suggested protoplanetary disks are less turbulent. However, previous studies have suggested that a low viscosity results in a long (> 10 Myr) disk lifetime if the disk evolves with only viscous accretion and PE. In this study, we investigated the effects of MHD winds and wind-driven accretion. First, we investigated the disk evolution with these MHD processes and (inefficient) viscous accretion. We found that although these MHD processes significantly change the inner disk structure, the disks last long. On the other hand, if we also consider PE, the disk lifetime can be less than 6 Myr. We conclude that all the three processes (i.e., PE, MHD winds, and wind-driven accretion) should be considered in a realistic disk evolutionary model.