Accretion onto supermassive black holes produces both the dramatic phenomena associated with active galactic nuclei and the underwhelming displays seen in the Galactic Center and most other nearby galaxies. I review selected aspects of the current theoretical understanding of black hole accretion, emphasizing the role of magnetohydrodynamic turbulence and gravitational instabilities in driving ...

Many models of gamma-ray bursts (GRBs) involve accretion onto a compact object, usually a black hole, at a mass accretion rate of order a fraction of a solar mass per second. If the accretion disk is larger than a few tens or hundreds of Schwarzschild radii, the accretion will proceed via a convection-dominated accretion flow (CDAF) in which most of the matter escapes to infinity rather than fa...

This paper presents a comprehensive quantitative baseline assessment of in situ net calcium carbonate accretion rates (g CaCO3 cm(-2) yr(-1)) of early successional recruitment communities on Calcification Accretion Unit (CAU) plates deployed on coral reefs at 78 discrete sites, across 11 islands in the central and south Pacific Oceans. Accretion rates varied substantially within and between isl...

We review the present knowledge of disk accretion in young low mass stars, and in particular, the mass accretion rate ˙ M and its evolution with time. The methods used to obtain mass accretion rates from ultraviolet excesses and emission lines are described, and the current best estimates of ˙ M for Classical T Tauri stars and for objects still surrounded by infalling envelopes are given. We ar...

We show that dwarf nova disks in quiescence have rather low magnetic Reynolds number, of order 10. Numerical simulations of magnetized accretion disks suggest that under these conditions magnetohydrodynamic turbulence and the associated angular momentum transport is sharply reduced. This could be the physical origin of episodic accretion in dwarf nova disks. If so, the standard disk instability...

We discuss the importance of accretion in calculating disk models for young stellar objects. In particular, we show that a disk inner rim, irradiated by both the star and the accretion shocks at the stellar surface, can naturally explain recent observations of DG Tau with the Keck interferometer. We present models for two objects, with mass accretion rates differing by almost two orders of magn...

The hydrodynamics of a variant of classical BondiHoyle-Lyttleton accretion is investigated: a totally absorbing sphere moves at various Mach numbers (3 and 10) relative to a medium, which is taken to be an ideal gas having a density gradient (of 3%, 20% or 100% over one accretion radius) perpendicular to the relative motion. I examine the influence of the Mach number, the adiabatic index, and t...

We reconstruct past accretion rates of a salt marsh on the island of Sylt, Germany, using measurements of the radioisotopes Pb and Cs, as well as historical aerial photographs. Results from three cores indicate accretion rates varying between 1 and 16 mm year. Comparisons with tide gauge data show that high accretion rates during the 1980s and 1990s coincide with periods of increased storm acti...

We consider the combined role of the thermal ionization, magneto-rotational and gravitational instabilities in thin accretion disks around supermassive black holes. We find that in the portions of the disk unstable to the ionization instability, the gas remains well coupled to the magnetic field even on the cold, neutral branch of the thermal limit cycle. This suggests that the ionization insta...

The radio source Sgr A at the center of our Galaxy is believed to be a 2.6× 10M⊙ black hole which accretes gas from the winds of nearby stars. We show that limits on the X-ray and infrared emission from the Galactic Center provide an upper limit of ∼ 8 × 10M⊙ yr −1 on the mass accretion rate in Sgr A. The advection-dominated accretion flow (ADAF) model favors a rate <∼ 10 M⊙ yr . In comparison,...