GLOBAL SIMULATIONS OF PROTOPLANETARY DISKS WITH OHMIC RESISTIVITY AND AMBIPOLAR DIFFUSION

Abstract

Protoplanetary disks are believed to accrete onto their central T Tauri star\nbecause of magnetic stresses. Recently published shearing box simulations\nindicate that Ohmic resistivity, ambipolar diffusion and the Hall effect all\nplay important roles in disk evolution. In the presence of a vertical magnetic\nfield, the disk remains laminar between 1-5au, and a magnetocentrifugal disk\nwind forms that provides an important mechanism for removing angular momentum.\nQuestions remain, however, about the establishment of a true physical wind\nsolution in the shearing box simulations because of the symmetries inherent in\nthe local approximation. We present global MHD simulations of protoplanetary\ndisks that include Ohmic resistivity and ambipolar diffusion, where the\ntime-dependent gas-phase electron and ion fractions are computed under FUV and\nX-ray ionization with a simplified recombination chemistry. Our results show\nthat the disk remains laminar, and that a physical wind solution arises\nnaturally in global disk models. The wind is sufficiently efficient to explain\nthe observed accretion rates. Furthermore, the ionization fraction at\nintermediate disk heights is large enough for magneto-rotational channel modes\nto grow and subsequently develop into belts of horizontal field. Depending on\nthe ionization fraction, these can remain quasi-global, or break-up into\ndiscrete islands of coherent field polarity. The disk models we present here\nshow a dramatic departure from our earlier models including Ohmic resistivity\nonly. It will be important to examine how the Hall effect modifies the\nevolution, and to explore the influence this has on the observational\nappearance of such systems, and on planet formation and migration.\n