We present optical spectra of 45 intermediate mass Herbig Ae/Be stars. Together with the multi-epoch spectroscopic and photometric data compiled for a large sample of these stars and ages estimated for individual stars by using pre-main sequence evolutionary tracks, we have studied the evolution of emission line activity in them. We find that, on average, the Hα emission line strength decreases...

We present a new perspective on the crystallinity of dust in protoplanetary disks. The dominant crystallization by thermal annealing happens in the very early phases of disk formation and evolution. Both the disk properties and the level of crystallinity are thereby directly linked to the properties of the molecular cloud core from which the star+disk system was formed. We show that, under the ...

We have investigated the phenomenon of explosive chromospheric evaporation from an accretion disk as a mechanism for fast variability in accreting sources such as low mass X-ray binaries and active galactic nuclei. This has been done in the context of advection dominated accretion flows, allowing both high and low states to be considered. This mechanism can in principle produce sub-millisecond ...

Radiatively inefficient (or advection dominated) disks are discussed at an introductory level. Ion supported and radiation supported flows are discussed, the different consequences of advection dominated flows onto black holes vs. solid surfaces (neutron stars, white dwarfs), hydrodynamics, the role of the ratio of specific heats, and the possible connection between ADAFs and outflows.

Two pseudo-Newtonian potentials, which approximate the angular and epicyclic frequencies of the relativistic accretion disk around rotating (and counter rotating) compact objects, are presented. One of them, the Logarithmically Modified Potential, is a better approximation for the frequencies while the other, the Second-order Expanded potential, also reproduces the specific energy for circular ...

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...

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...

A geometrically thin, optically thick, warped accretion disk with a central source of luminosity is subject to non-axisymmetric forces due to radiation pressure; the resulting torque acts to modify the warp. Initially planar accretion disks are unstable to warping driven by radiation torque, as shown in a local analysis by Pringle (1996) and a global analysis of the stable and unstable modes by...

It is well known that the outer parts of QSO accretion disks are prone to selfgravity if heated solely by orbital dissipation. Such disks might be expected to form stars rather than accrete onto the black hole. The arguments leading to this conclusion are reviewed. Conversion of a part of the gas into high-mass stars or stellar-mass black holes, and the release of energy in these objects by fus...

Observations indicate that mass accretion rates onto low-mass protostars are generally lower than the rates of infall to their disks; this suggests that much of the protostellar mass must be accreted during rare, short outbursts of rapid accretion. We explore when protostellar disk accretion is likely to be highly variable. While constant α disks can in principle adjust their accretion rates to...