Despite of a lot of efforts and interesting results, accretion disk theory is still under development and there are several unknowns: specifically, all available models are in the steady state approximation. In this letter, we investigate unsteady motion, beginning with shock wave–turbulence interaction. Indeed, unsteady flow allows this type of interaction, which in turn results in substantial...

I review the current state of knowledge regarding disk accretion in young brown dwarfs (BDs), and the interaction of the disk with the central object. In particular, I discuss (1) observations of accretion/outflow phenomena in BDs; (2) techniques for measuring accretion rates (Ṁacc ); (3) the dependence of Ṁacc on the central mass from stars to brown dwarfs; (4) the temporal evolution of Ṁacc ;...

We present evidence of magnetically-mediated disk accretion in Herbig Ae/Be stars. Magnetospheric accretion models of Balmer and sodium profiles calculated with appropriate stellar and rotational parameters are in qualitative agreement with the observed profiles of the Herbig Ae star UX Ori, and yield a mass accretion rate of ∼ 10 M⊙ yr. If more recent indications of an extremely large rotation...

In the absence of direct kinematic measurements, the mass of an accreting black hole is sometimes inferred from the X-ray spectral parameters of its accretion disk; specifically, from the temperature and normalization of a disk-blackbody model fit. Suitable corrections have to be introduced when the accretion rate approaches or exceeds the Eddington limit. We summarize phenomenological models t...

We present global solutions of optically thin, two-temperature black hole accretion disks incorporating magnetic fields. We assume that the ̟φ-component of the Maxwell stress is proportional to the total pressure, and prescribe the radial dependence of the magnetic flux advection rate in order to complete the set of basic equations. We obtained the magnetically supported (low-β) disk solutions, ...

We fit outbursts of two X-ray novae (Nova Monocerotis 1975 = A 0620–00 and Nova Muscae 1991 = GS 1124–683) using a time-dependent accretion disk model. The model is based on a new solution for a diffusion-type equation for the non-steady-state accretion and describes the evolution of a viscous α–disk in a binary system after the peak of an outburst, when matter in the disk is totally ionized. T...

Using a new generalized second law of thermodynamics, the information and entropy of a black hole and its accretion disk are analyzed respectively. We find the bound of the information channel width of a black hole, which is determined by the variation rate of the horizon temperature and the mass of the black hole’s shell (the accretion disk close to the horizon).

The conventional picture of disk accretion onto magnetized neutron stars has been challenged by the spin changes observed in a few X-ray pulsars, and by theoretical results from numerical simulations of diskmagnetized star interactions. These indicate possible accretion during the propeller regime and the spin-down torque increasing with the accretion rate. Here we present a model for the accre...

The effect of a possible hydrodynamical wind on the nature of hot accretion disk solutions is studied. It is found that the advection dominated branch, in the presence of a wind, maintains the self-similar solution for the disk structure with the temperature, θ ∝ 1/r, optical depth τ ∝ r and accretion rate ṁ ∝ r . Based on global solutions cooling due to wind energy loss and advection are found...

Accretion disks are observed in young stars, cataclysmic variables, binary X-ray sources et al. Accretion disk theory was first developed as a theory with the local heat balance, where the whole energy produced by a viscous heating was emitted to the sides of the disk. Important part of this theory was the phenomenological treatment of the turbulent viscosity, known the “ alpha” prescription, w...