MRI in Accretion Disks: Where Is It Actually Active?
Subhanjoy Mohanty, Imperial College London
Abstract: In the standard picture of layered accretion disks, MRI-driven accretion is expected everywhere except in a Dead Zone near the mid-plane in the inner disk regions, where the ionization fraction is too low. However, 3 aspects have remained largely unintegrated into such disk models: (1) the size of the Dead Zone in the presence of dust grains (which soak up charge); (2) the effect of ambipolar diffusion (which becomes increasingly important as the gas density decreases with height and radius); and (3) the change in disk properties as one moves from solar-type stars to very low-mass M dwarfs (which make up 80% of the galactic stellar population). I will briefly discuss new models we have constructed taking grain and ambipolar diffusion effects into account, for disks around both solar-type and very low-mass stars. The models suggest that a very significant portion of stellar disks is in fact rendered MRI-inactive by the combined effect of grains and ambipolar diffusion; they also suggest important changes in the size of the Dead Zone with changing stellar mass. The results have strong consequences for both disk accretion as well as planet formation.