Oded Regev

Thermal equilibria of accretion disks

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.

Dynamics of fronts in thermally bistable fluids

Fronts between different thermal phases of a fluid, when the cooling function allows two thermally stable phases around an unstable one (bistability), are investigated. Fluid motion is included, in addition to thermal conduction. For a one-dimensional case the front dynamics are investigated by introducing an appropriate Lyapunov functional. It is assumed that the coefficient of thermal conductivity is small, so that the front thickness is very small compared with front separations. Pairs of adjacent fronts define a cloud (or an intercloud region), and their motion gives rise to the growth of the cloud (condensation) or of the intercloud region (evaporation). The properties of various types of fronts separating the different thermal phases of the fluid are discussed. Interaction between fronts and its effect on front motion is found using an approximation method

Accretion onto hot white dwarfs in relation to symbiotic novae

Numerical calculations are used to study the hydrodynamic evolution of a hot white dwarf with 1 solar mass accreting hydrogen-rich matter at rates between 10 to the -8th and 10 to the -6th solar masses/yr. It is found that for accretion at a rate of about 10 to the -8th solar masses/yr, nova-type outbursts of long duration occur at intervals of about 1500 yr. About half of the accreted envelope is ejected during these outbursts. At a rate of about 10 to the -7th solar masses/yr, the star alternates between comparable periods at a high plateau luminosity and giant dimensions and periods at a low luminosity and white dwarf dimension. At 10 to the -6th solar masses/yr, equilibrium is achieved with a typical red giant luminosity supported by steady hydrogen burning. It is concluded that symbiotic novae are more likely to occur in detached systems involving wind accretors. Thus, the contribution of symbiotic stars to the frequency of type I supernovae is severely constrained. 39 refs.

Global transient dynamics of three-dimensional hydrodynamical disturbances in a thin viscous accretion disk

Thin viscous Keplerian accretion disks are considered asymptotically stable, even though they can show significant dynamic activity on short time scales. In this paper the dynamics of nonaxisymmetric hydrodynamical disturbances of disks are investigated analytically, building upon the steady state three-dimensional structure and evolution of axisymmetric perturbations explored in previous work. Assuming a polytropic equation of state, solutions are found by means of an asymptotic expansion in the small parameter measuring the ratio of the disk thickness to characteristic radius. In-depth analysis shows that every perturbation that disturbs the radial velocity induces significant transient growth in the (acoustic) energy of the evolving disturbance. This effect is most evident in the density and vertical velocity. The transient growth observed is tied to the nonseparable nature of the solutions where, in particular, pattern evolution is controlled by a similarity variable composed of the radial coordinate ...

Disturbed FLIERs in Planetary Nebulae

We examine the recent claim by Balick et al. that their observations of fast low-ionization emission regions in four planetary nebulae pose serious problems to the so-called fast-bullet model for the FLIERs. Using timescale estimates and the results of existing numerical studies, and from the phenomenology of 11 objects, we conclude that, contrary to the claim made in the above paper, the fast-bullet model remains consistent with observations.

Diamagnetic Blob Interaction Model of T Tauri Variability

Assuming a diamagnetic interaction between a stellar-spot originated localized magnetic field and gas blobs in the accretion disk around a T- Tauri star, we show the possibility of ejection of such blobs out of the disk plane. Choosing the interaction radius and the magnetic field parameters in a suitable way gives rise to closed orbits for the ejected blobs. A stream of matter composed of such blobs, ejected on one side of the disk and impacting on the other, can form a hot spot at a fixed position on the disk (in the frame rotating with the star). Such a hot spot, spread somewhat by disk shear before cooling, may be responsible in some cases for the lightcurve variations observed in various T-Tauri stars over the years. An eclipse-based mechanism due to stellar obscuration of the spot is proposed. Assuming high disk inclination angles it is able to explain many of the puzzling properties of these variations. By varying the field parameters and blob initial conditions we obtain variations in the apparent angular velocity of the hot spot, producing a constantly changing period or intermittent periodicity disappearance in the models.Assuming a diamagnetic interaction between a stellar spot-originated localized magnetic field and gas blobs in the accretion disk around a T Tauri star, we show the possibility of ejection of such blobs out of the disk plane. Choosing the interaction radius and the magnetic field parameters in a suitable way gives rise to closed orbits for the ejected blobs. A stream of matter composed of such blobs, ejected on one side of the disk and impacting on the other, can form a hot spot at a fixed position on the disk (in the frame rotating with the star). Such a hot spot, spread somewhat by disk shear before cooling, may be responsible in some cases for the light-curve variations observed in various T Tauri stars over the years. An eclipse-based mechanism due to stellar obscuration of the spot is proposed. With the assumption of high disk inclination angles, it is able to explain many of the puzzling properties of these variations. By varying the field parameters and blob initial conditions, we obtain variations in the apparent angular velocity of the hot spot, producing a constantly changing period or intermittent periodicity disappearance in the models.

Kelvin-Helmholtz instability in clusters of galaxies

The motion of galaxies through the hot intracluster gas is examined. It is found that a Kelvin-Helmholtz instability develops at the interface between the moving galaxy and the gas. Its effect is gas stripping from the moving galaxy: the rate of stripping is estimated. The role of viscosity in this process is discussed.

Tidal disruption of stars by a massive black hole

The hydrodynamics of stellar models in the tidal field of a massive black hole is calculated numerically under certain simplifying assumptions. It is found that solar type stars are totally disrupted only when falling toward a black hole whose mass does not exceed 106–107M⊙. Red giants lose their entire envelope for all black hole masses studied.Our findings strengthen the view that tidal disruption is not an important source of gas in AGNs.

Linear and Nonlinear Incompressible Waves

Waves in fluids are ubiquitous, from the outward propagation of pond ripples, ocean waves crashing ashore, sound and shocks in the air, and so many others that to list them all would be impossible.

Modern Fluid Dynamics for Physics and Astrophysics

This book grew out of the need to provide students with a solid introduction to modern fluid dynamics. It offers a broad grounding in the underlying principles and techniques used, with some emphasis on applications in astrophysics and planetary science. The book comprehensively covers recent developments, methods and techniques, including, for example, new ideas on transitions to turbulence (via transiently growing stable linear modes), new approaches to turbulence (which remains the enigma of fluid dynamics), and the use of asymptotic approximation methods, which can give analytical or semi-analytical results and complement fully numerical treatments. The authors also briefly discuss some important considerations to be taken into account when developing a numerical code for computer simulation of fluid flows. Although the text is populated throughout with examples and problems from the field of astrophysics and planetary science, the text is eminently suitable as a general introduction to fluid dynamics. It is assumed that the readers are mathematically equipped with a reasonable knowledge in analysis, including basics of ordinary and partial differential equations and a good command of vector calculus and linear algebra. Each chapter concludes with bibliographical notes in which the authors briefly discuss the chapters essential literature and give recommendations for further, deeper reading. Included in each chapter are a number of problems, some of them relevant to astrophysics and planetary science. The book is written for advanced undergraduate and graduate students, but will also prove a valuable source of reference for established researchers