We consider accretion disks consisting of counter-rotating gaseous components with an intervening shear layer. Configurations of this type may arise from the accretion of newly supplied counter-rotating gas onto an existing co-rotating gas disk. For simplicity we consider the case where the gas well above the disk midplane is rotating with angular rate +Ω and that well below has the same proper...

Current stellar evolution models predict that during the core collapse of massive stars, a considerable amount of the stellar material will fall back onto the compact, collapsed remnants (neutron stars or black holes), usually in the form of an accretion disk. This triggers rapid mass accretion onto, e.g., the black hole, and produces energetic explosions known as Gamma-ray bursts. However it i...

We briefly review recent developments in black hole accretion disk theory, emphasizing the vital role played by magnetohydrodynamic (MHD) stresses in transporting angular momentum. The apparent universality of accretion-related outflow phenomena is a strong indicator that large-scale MHD torques facilitate vertical transport of angular momentum. This leads to an enhanced overall rate of angular...

We reexamine the hypothesis that the optical/UV/soft X-ray continuum of Active Galactic Nuclei is thermal emission from an accretion disk. Previous studies have shown that fitting the spectra with the standard, optically thick and geometrically thin accretion disk models often led to luminosities which contradict the basic assumptions adopted in the standard model. There is no known reason why ...

Young stellar systems orbiting in the potential of their birth cluster can accrete from the dense molecular interstellar medium during the period between the star’s birth and the dispersal of the cluster’s gas. Over this time, which may span several Myr, the amount of material accreted can rival the amount in the initial protoplanetary disk; the potential importance of this ‘tail-end’ accretion...

We have performed the first measures of mass accretion rates in the core of the Orion Nebula Cluster. Four adjacent fields centered on the Trapezium stars have been imaged in the Uand B-bands using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. We obtained photometry for 91 stars in the U-band (F336W) and 71 stars in the B-band (F439W). The WFPC2 archive was also searche...

It is now accepted that accretion onto classical T Tauri stars is controlled by the stellar magnetosphere, yet to date most accretion models have assumed that their magnetic fields are dipolar. By considering a simple steady state accretion model with both dipolar and complex magnetic fields we find a correlation between mass accretion rate and stellar mass of the form Ṁ ∝ Mα ∗ , with our resul...

Multiwavelength observations of the black hole X-ray binary XTE J1118+480 have offered abundant spectral and timing information about the source and have thus provided serious challenges to theoretical models. We propose a coupled accretion-jet model to interpret the observations. We model the accretion flow as an outer standard thin accretion disk truncated at a transition radius by an inner h...

We revisit the flip-flop instability of two-dimensional planar accretion using high-fidelity numerical simulations. By starting from an initially steady-state axisymmetric solution, we are able to follow the growth of this overstability from small amplitudes. In the small-amplitude limit, before any transient accretion disk is formed, the oscillation period of the accretion shock is comparable ...