How do T Tauri disks accrete? The magneto-rotational instability (MRI) supplies one means, but protoplanetary disk gas is typically too poorly ionized to be magnetically active. Here we show that the MRI can, in fact, explain observed accretion rates for the sub-class of T Tauri disks known as transitional systems. Transitional disks are swept clean of dust inside rim radii of ∼10 AU. Stellar c...

Geometrically thick disks may form after tidal disruption events, and rapid accretion may lead to short flares followed by long-term, lower-level emission. Using a novel accretion disk code which relies primarily on global conservation laws and the assumption that viscosity is everywhere positive, a broad range of physically allowed evolutionary sequences of thick disks is investigated. The mai...

I review recent numerical studies of accretion disks, focusing on measurements of the turbulent shear stress, or α, in the shearing box model. I conclude with a list of astronomically relevant open questions that can be settled via future numerical experiments.

I review recent advances in our understanding of accretion disks in transient systems− the dwarf novae and the soft X-ray transients. The primary theme will be the ongoing development of theory in response to the observations. The accretion disk limit cycle model appears to provide a unifying framework within which we may begin to understand what is seen in different types of interacting binary...

Turbulent viscosity in cold accretion disks is likely to be hydrodynamic in origin. We investigate the growth of hydrodynamic perturbations in a small region of a disk, which we model as a linear shear flow with Coriolis force, between two parallel walls. Although there are no exponentially growing eigenmodes in this system, because of the non-normal nature of the modes, it is possible to have ...

We test approximate approaches to solving a neutrino transport problem that presents itself in the analysis of some accretion-disk models. Approximation # 1 consists of replacing the full, angulardependent, distribution function by a two-stream simulation, where the streams are respectively outwardly and inwardly directed, with angles cos θ = ±1/ √ 3 to the vertical. In this approximation the f...

We show that most of hot, optically thin accretion disk models which ignore advective cooling are not self-consistent. We have found new types of optically thin disk solutions where cooling is dominated by radial advection of heat. These new solutions are thermally and viscously stable. Subject headings: accretion, accretion disks | instabilities

We present a set of new characteristic frequencies associated with accretion disks around compact objects. These frequencies arise from persistent rotating patterns in the disk that are finite in radial extent and driven purely by the gravity of the central body. Their existence depends on general relativistic corrections to orbital motion and, if observed, could be used to probe the strong gra...

The jets observed to emanate from many compact accreting objects may arise from the twisting of a magnetic field threading a differentially rotating accretion disk which acts to magnetically extract angular momentum and energy from the disk. Two main regimes have been discussed, hydromagnetic jets, which have a significant mass flux and have energy and angular momentum carried by both matter an...