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.

Powerpoint presentation