Giora Shaviv

Convective Overshooting in Stellar Interior Models

ABS>The question of overshooting from interior convective cores in stellar models is examined in the framework of a simplified nonlocal mixing- length model for convection. The most important results are (a) a finite nonnegligible overshoot is obtained for parameter combinations typical of stellar convective cores; (b) a finite nonzero overshoot is obtained even in the limit of adiabatic convection (achieved when convective speeds tend to zero). A general thermedynamic argument is presented to show that the limiting results are not model dependent.

Thermal instability in accretion flows onto degenerate stars

We report the discovery of a new kind of thermal instability in accretion flows onto stellar atmospheres. The instability results in periodic variations in the height of the standoff shock which forms above the stellar surface. The hard X-ray temperature and luminosity vary with this period, as does any other radiation produced in the hot gas between the shock and the surface. Conditions for the appearance of this instability are most favorable in accreting white dwarf stars, such as cataclysmic variables and AM Her stars. We suggest that a search for such variability be carried out in AM Her stars in both the hard X-ray and UN-optical bands. The resulting period, which is of order 1--100 s for AM Her stars, permits the determination of the accretion rate per unit area at the stellar surface.

STRUCTURE OF STEADY STATE ACCRETION SHOCKS WITH SEVERAL COOLING FUNCTIONS : CLOSED INTEGRAL-FORM SOLUTION

We present, for the first time, a closed integral-form solution to the accretion shock structures for the case where the cooling is due to optically thin bremsstrahlung emission and a series of power-law cooling functions of density and temperature. Our results can provide useful checks on numerical calculations and simple accurate estimates for valuable parameters such as the shock height. For the case where the cooling rate j = (2/3)Arho(exp 2)(P/rho)(exp 1/2)(1 + epsilon (sub s)(P/P(sub s)(exp alpha)(rho(sub s)/rho)(exp beta)), we find that a substantial amount of the accretion energy is released at the base of the accretion shock in the form of bremsstrahlung radiation. This implies that for a cyclotron-dominated shock (qualitatively given by alpha = 2.0, beta = 3.85, and epsilon(sub s) is much greater than 1), bremsstrahlung cooling still plays a crucial role in determining the shock structure. Our results are shown to be consistent with detailed numerical calculations.

A model for the 1987 outburst of the recurrent Nova U Scorphii

The paper presents simulations of the outburst of U Sco by accreting material, with a solar abundance of the CNO nuclei, at a rate of 1.1 x 10 to the -6th solar mass/yr onto a 1.35-solar mass white dwarf. This evolutionary sequence takes about 2.6 yr to reach runaway conditions and ultimately eject 4 x 10 to the -7th solar mass moving with speeds exceeding about 400 km/s. It is found that ejection occurred by radiation pressure rather than by explosive CNO burning. 22 references.

Is There a Dynamic Effect in the Screening of Nuclear Reactions in Stellar Plasmas

We consider a fully ionized hydrogen plasma and investigate the two following questions. Does an ions kinetic energy affect its electrostatic potential energy when it is in statistical equilibrium with a plasma? In other words, is there a difference between the potential energy of a test particle moving through a plasma and the potential energy of a particle in a statistical equilibrium? Are the plasma effects on the rate of nuclear reactions dependent on the kinetic energy of the reacting particles? Carraro and coworkers assumed that the potentials of a test particle and of a particle in a statistical equilibrium are identical and applied the potential of a test particle to evaluate the effect of the plasma on nuclear reactions. However, criticism raised recently by several authors, including Weneser and Gruzinov, claims that the two potentials are not identical, and hence the potential of a test particle, which depends on its kinetic energy, cannot be applied to evaluate the effect of the plasma on nuclear reactions in cores of stars. We examine the relevant statistical mechanics assumptions and in view of the inexistence of a complete rigorous proof, check them numerically. We find the following. The long-term average potential energy of a particle in a statistical equilibrium with the plasma is independent of the particles kinetic energy, at least for particles with kinetic energies of up to 10kT (which is at present our numerical limit). The potential energy of a test particle differs from the long-term average potential energy of a particle in a statistical equilibrium. We define the plasma effect on the nuclear reaction rate and calculate it directly. We find that it depends on the relative kinetic energy of the interacting particles. Using the same numerical calculation, we find the dependence of a particles potential energy in a statistical equilibrium on its kinetic energy and the potential of a test particle and the dependence of the plasma effects on the relative kinetic energy of the interacting particles. We conclude that a dynamic effect on the screening does exist. In view of our results, we discuss the possibility of measuring it in the laboratory and clarify the difference between reactions in a plasma in thermal equilibrium and a laboratory scattering experiment. The frequently made assumption that a test particle can be assumed in evaluating the kernel of plasma kinetic equations is found to be unjustified. We show that, under the conditions prevailing in the solar core, the mean field approximations are not sufficient to describe the screening, and fluctuations play a crucial role.

Time-dependent accretion onto magnetic white dwarfs: Effects of cyclotron emission

Previous accretion models have shown that if the cooling is due to optically thin bremsstrahlung, the flow is unstable. The shock height oscillates with a period approx.1 s, which corresponds to the cooling time for parameters characteristic of the AM Her binaries. Here we show that the effects of cyclotron cooling stabilize the flow if the magnetic field is large enough. For a 0.5 M/sub sun/ white dwarf accreting 1 g cm S s , the flow is stable if B is more than or approximately equal to 2 x 10X gauss. We also show that the bremsstrahlung X-ray emission and the optical cyclotron emission are 180 out of phase when oscillations are present. The relevance of these calculations to the observed optical oscillations, with periods of approx.1 s, in two AM Her stars is discussed.

Global axisymmetric dynamics of thin viscous accretion disks

The purpose of this paper is to explore the steady state and dynamical behavior of thin, axisymmetric, viscous accretion disks. To facilitate an analytical treatment we replace the energy equation with a general polytropic assumption. The asymptotic expansion of Kluzniak & Kita (2000, Three-dimensional structure of an alpha accretion disk (arXiv:astro-ph/0006266)), which extended the method of Regev (1983, A&A, 126, 146) to a full steady polytropic disk (with n = 3/2), is further developed and implemented for both the steady (for any polytropic index) and time-dependent problems. The spatial form and temporal behavior of selected dynamical disturbances are studied in detail. It is shown that the transient dynamics resulting from initial perturbations on the linearly stable steady state gives rise to substantial growth of perturbations. We identify the initial perturbation space which leads to such transient growth and provide analytical solutions which manifest this behavior three terms (physical causes) responsible for the appearance of transient dynamics are identified. Two depend explicitly on the viscosity while the third one is relevant also for inviscid disks. The main conclusion we draw is that transient dynamics and, in particular, significant perturbation energy amplification occurs in disks on a global scale. We speculate on the possible implications of these findings to accretion disk theory.

THE ELECTROSTATIC SCREENING OF NUCLEAR REACTIONS IN THE SUN

We apply the method of Shaviv & Shaviv to evaluate from first principles the plasma screening correction to the rate of nuclear reactions under conditions similar to those prevailing in the present-day Sun. We calculate the screening factor for the p-p chain and the CN cycle nuclear reactions. We find the following:

The formation of Al-26 in nova explosions

The amount of 26 Al in the interstellar medium, which has been detected both by the HEAO 3 γ-ray spectrometer and the SMM experiment, in combination with the measured overabundance of 26 Mg in meteorites, is predicted to have been produced by nucleosynthesis in nova explosions. To test this prediction, the nucleosynthesis of 26 Al is investigated during hot hydrogen burning along temperature-density-time profiles obtained from hydrodynamic simulations of nova outbursts.

Properties of quasi-periodic oscillations in accreting magnetic white dwarfs

Previous studies of time-dependent accretion onto magnetic white dwarfs, in which the cooling was assumed to be due to bremsstrahlung emission, have shown that the accretion shock undergoes oscillations. However, when cyclotron cooling is also included, the oscillations are damped for sufficiently strong magnetic fields. Here we demonstrate that the oscillations can be sustained by accretion-fluctuation-induced excitations. The frequency of the QPOs are shown to increase quadratically with the magnetic field strength. We interpret the oscillations as a two-phase process in which bremsstrahlung cooling dominates in one half-cycle and cyclotron cooling in the other. Such a process may have very different consequences compared to a single-phase process where the functional form of the cooling is essentially the same throughout the cycle. If in the two-phase process damping occurs mainly in the cyclotron cooling half-cycle, there will be a universal effective damping factor which tends to suppress all oscillation modes indiscriminately. The oscillations of the accretion shock also could be a limit cycle process in which the system vacillates between two branches. 35 refs.