T. Plewa

A comparative study of disc–planet interaction

We perform numerical simulations of a disc-planet system using various grid-based and smoothed particle hydrodynamics (SPH) codes. The tests are run for a simple setup where Jupiter and Neptune mass planets on a circular orbit open a gap in a protoplanetary disc during a few hundred orbital periods. We compare the surface density contours, potential vorticity and smoothed radial profiles at several times. The disc mass and gravitational torque time evolution are analysed with high temporal resolution. There is overall consistency between the codes. The density profiles agree within about 5 per cent for the Eulerian simulations. The SPH results predict the correct shape of the gap although have less resolution in the low-density regions and weaker planetary wakes. The disc masses after 200 orbital periods agree within 10 per cent. The spread is larger in the tidal torques acting on the planet which agree within a factor of 2 at the end of the simulation. In the Neptune case, the dispersion in the torques is greater than for Jupiter, possibly owing to the contribution from the not completely cleared region close to the planet.

Pulsar Recoil by Large-Scale Anisotropies in Supernova Explosions

Assuming that the neutrino luminosity from the neutron star core is sufficiently high to drive supernova explosions by the neutrino-heating mechanism, we show that low-mode (l=1,2) convection can develop from random seed perturbations behind the shock. A slow onset of the explosion is crucial, requiring the core luminosity to vary slowly with time, in contrast to the burstlike exponential decay assumed in previous work. Gravitational and hydrodynamic forces by the globally asymmetric supernova ejecta were found to accelerate the remnant neutron star on a time scale of more than a second to velocities above 500 km s(-1), in agreement with observed pulsar proper motions.

On validating an astrophysical simulation code

We present a case study of validating an astrophysical simulation code. Our study focuses on validating FLASH, a parallel, adaptive-mesh hydrodynamics code for studying the compressible, reactive flows found in many astrophysical environments. We describe the astrophysics problems of interest and the challenges associated with simulating these problems. We describe methodology and discuss solutions to difficulties encountered in verification and validation. We describe verification tests regularly administered to the code, present the results of new verification tests, and outline a method for testing general equations of state. We present the results of two validation tests in which we compared simulations to experimental data. The first is of a laser-driven shock propagating through a multilayer target, a configuration subject to both Rayleigh-Taylor and Richtmyer-Meshkov instabilities. The second test is a classic Rayleigh-Taylor instability, where a heavy fluid is supported against the force of gravity by a light fluid. Our simulations of the multilayer target experiments showed good agreement with the experimental results, but our simulations of the Rayleigh-Taylor instability did not agree well with the experimental results. We discuss our findings and present results of additional simulations undertaken to further investigate the Rayleigh-Taylor instability.

Modeling W44 as a Supernova Remnant in a Density Gradient with a Partially Formed Dense Shell and Thermal Conduction in the Hot Interior. II. The Hydrodynamic Models

We show that many observations of W44, a supernova remnant in the Galactic plane at a distance of about 2500 pc, are remarkably consistent with the simplest realistic model. The model remnant is evolving in a smooth ambient medium of fairly high density, about 6 cm-3 on average, with a substantial density gradient. At the observed time it has an age of about 20,000 yr, consistent with the age of the associated pulsar, and a radius of 11-13 pc. Over most of the outer surface, radiative cooling has become important in the postshock gas; on the denser end there has been sufficient compression of the cooled gas to develop a very thin dense half-shell of about 450 M☉, supported against further compression by nonthermal pressure. The half-shell has an expansion velocity of about 150 km s-1 and is bounded on the outer surface by a radiative shock with that speed. The deep interior of the remnant has a substantial and fairly uniform pressure, as expected from even highly idealized adiabatic models; our model, however, is never adiabatic. Thermal conduction, while the remnant is young and hot, reduces the need for expansion cooling and prevents formation of the intensely vacuous cavity characteristic of adiabatic evolution. It radically alters the interior structure from what one might expect from familiarity with the Sedov solution. At the time of observation, the temperature in the center is about 6 × 106 K, the density about 1 cm-3. The temperature decreases gradually away from the center, while the density rises. Farther out, where cooling is becoming important, the pressure drops precipitously, and the temperature in the denser gas there is quite low. We provide several analytic tools for the assembly of models of this type. We review the early evolution and shell formation analyses and their generalizations to evolution in a density gradient. We also calculate the density and temperature that should be present in the hot interior of a remnant with thermal conduction. We supply the van der Laan mechanism in a particularly useful form for the calculation of radio continuum from radiative remnants. Finally, we estimate the optical emission that should be present from fluorescence of UV light, emitted by the forming shell and the radiative shock and absorbed in the cold shell and the ambient medium, and the associated 63 μm [O I] emission. Both are in agreement with the intensity and spatial structures found in recent observations. Neither requires interaction with a dense molecular cloud for its generation. We calculate the gamma rays that should be emitted by cosmic-ray electrons and ions in the shell, interacting with the cold material, and find each capable of generating about 25% of the flux reported by EGRET for the vicinity.

Type Ia supernova explosion: Gravitationally confined detonation

We present a new mechanism for Type Ia supernova explosions in massive white dwarfs. The scenario follows from relaxing assumptions of symmetry and involves a detonation born near the stellar surface. The explosion begins with an essentially central ignition of a deflagration that results in the formation of a buoyancy-driven bubble of hot material that reaches the stellar surface at supersonic speeds. The bubble breakout laterally accelerates fuel-rich outer stellar layers. This material, confined by gravity to the white dwarf, races along the stellar surface and is focused at the location opposite to the point of the bubble breakout. These streams of nuclear fuel carry enough mass and energy to trigger a detonation just above the stellar surface that will incinerate the white dwarf and result in an energetic explosion. The stellar expansion following the deflagration redistributes mass in a way that ensures production of intermediate-mass and iron group elements with ejecta having a strongly layered structure and a mild amount of asymmetry following from the early deflagration phase. This asymmetry, combined with the amount of stellar expansion determined by details of the evolution (principally the energetics of deflagration, timing of detonation, and structure of the progenitor), can be expected to create a family of mildly diverse Type Ia supernova explosions.

The Piecewise Parabolic Method for Multidimensional Relativistic Fluid Dynamics

We present an extension of the piecewise parabolic method to special relativistic fluid dynamics in multidimensions. The scheme is conservative, dimensionally unsplit, and suitable for a general equation of state. Temporal evolution is second-order accurate and employs characteristic projection operators; spatial interpolation is piecewise parabolic making the scheme third-order accurate in smooth regions of the flow away from discontinuities. The algorithm is written for a general system of orthogonal curvilinear coordinates and can be used for computations in non-Cartesian geometries. A nonlinear iterative Riemann solver based on the two-shock approximation is used in flux calculation. In this approximation, an initial discontinuity decays into a set of discontinuous waves only implying that, in particular, rarefaction waves are treated as flow discontinuities. We also present a new and simple equation of state that approximates the exact result for the relativistic perfect gas with high accuracy. The strength of the new method is demonstrated in a series of numerical tests and more complex simulations in one, two, and three dimensions.

Nucleosynthesis and Clump Formation in a Core-Collapse Supernova

High-resolution two-dimensional simulations were performed for the first 5 minutes of the evolution of a core-collapse supernova explosion in a 15 M middle dot in circle blue supergiant progenitor. The computations start shortly after bounce and include neutrino-matter interactions by using a lightbulb approximation for the neutrinos and a treatment of the nucleosynthesis due to explosive silicon and oxygen burning. We find that newly formed iron-group elements are distributed throughout the inner half of the helium core by Rayleigh-Taylor instabilities at the (Ni + Si)/O and (C + O)/He interfaces, seeded by convective overturn during the early stages of the explosion. Fast-moving nickel mushrooms with velocities up to approximately 4000 km s-1 are observed. This offers a natural explanation for the mixing required in light-curve and spectral synthesis studies of Type Ib explosions. A continuation of the calculations to later times, however, indicates that the iron velocities observed in SN 1987A cannot be reproduced because of a strong deceleration of the clumps in the dense shell left behind by the shock at the He/H interface.

Hybrid Characteristics: 3D radiative transfer for parallel adaptive mesh refinement hydrodynamics

Received * / Accepted * Abstract. We have developed a three-dimensional radiative transfer method designed specifically for use with parallel adaptive mesh refinement hydrodynamics codes. This new algorithm, wh ich we call hybrid characteristics, introduces a novel form of ray tracing that can neither be classified as long, nor as shor t characteristics, but which applies the underlying princi ples, i.e. efficient execution through interpolation and paralleliza bility, of both. Primary applications of the hybrid characteristics method are radiation hydrodynamics problems that take into account the effects of photoionization and heating due to point sources of radiation. The method is implemented in the hydrodynamics package FLASH. The ionization, heating, and cooling processes are modelled using the DORIC ionization package. Upon comparison with the long characteristics method, we find tha t our method calculates the column density with a similarly high accuracy and produces sharp and well defined shadows. We show the quality of the new algorithm in an application to the photoevaporation of multiple over-dense clumps. We present several test problems demonstrating the feasibility of our method for performing high resolution three-dimensional radiation hydrodynamics calculations that span a large range of scales. Initial performance tests show that the ray tra cing part of our method takes less time to execute than other parts of the calculation (e.g. hydrodynamics and adaptive mesh refinem ent), and that a high degree of efficiency is obtained in parallel ex ecution. Although the hybrid characteristics method is developed for problems involving photoionization due to point sources, the algorithm can be easily adapted to the case of more general radiation fields.

Modeling W44 as a Supernova Remnant in a Density Gradient, with a Partially Formed Dense Shell and Thermal Conduction in the Hot Interior

We show that many observations of W44, a supernova remnant in the galactic plane at a distance of about 2500 pc, are remarkably consistent with the simplest realistic model. The model remnant is evolving in a smooth ambient medium of fairly high density, about 6 cm −3 on average, with a substantial density gradient. At the observed time it has an age of about 20,000 years, consistent with the age of the associated pulsar, and a radius of 11 to 13 pc. Over most of the outer surface, radiative cooling has become important in the post shock gas; on the denser end there has been sufficient compression of the cooled gas to develop a very thin dense half shell of about 450 M⊙ , supported against further compression by nonthermal pressure. The half shell has an expansion velocity of about 150 km s −1 , and is bounded on the outer surface by a radiative shock with that speed. The deep interior of the remnant has a substantial and fairly uniform pressure, as expected from even highly idealized adiabatic models; our model, however, is never adiabatic. Thermal conduction, while the remnant is young and hot, reduces the need for expansion cooling, and prevents formation of the intensely vacuous cavity characteristic of adiabatic evolution. It radically alters the interior structure from what one might expect from familiarity with the Sedov solution. At the time of observation, the temperature in the center is about 6×10 6 K, the density about 1 cm −3 . The temperature decreases gradually away from the center, while the density rises. Farther out where cooling is becoming important, the pressure drops precipitously and the temperature in the denser gas there is quite low. Our model is similar to but more comprehensive than the recent one by Harrus et al. (1997). Because their model lacked thermal conduction, ours is more successful in providing the thermal x-rays from the hot interior, including a better match to the spectrum, but neither provides the sharpness of the central peaking without further complications. By using a 2d hydrocode to follow the evolution in a density gradient, we are able to verify that the spatial and velocity structure of the HI shell are a good match to the observations, without the complications suggested by Koo and Heiles (1995), and to demonstrate that the remnant’s asymmetry does not substantially affect the distribution of x-ray emitting material. A 1d hydrocode model is then used to explore the effects of nonequilibrium ionization on the x-ray spectrum and intensity. We calculate the radio continuum emission expected from the compression of the ambient magnetic field and cosmic rays into the dense shell (the van der Laan mechanism, 1962a) and find it to be roughly consistent with observation, though the required density of ambient cosmic ray electrons is about 4 times greater than that estimated for the solar neighborhood. We estimate the optical emission that should be present from fluorescence of UV, emitted by the forming shell and the radiative shock and absorbed in the cold shell and the ambient medium, and the associated 63 µm [OI] emission. Both are in agreement with the intensity and spatial structures found in recent observations. Neither requires interaction with a dense molecular cloud for its generation. We calculate the gamma rays that should be emitted by cosmic ray electrons and ions in the shell, interacting with the cold material, and find each capable of generating about 25% of the flux reported by EGRET for the vicinity. We provide several analytic tools for the assembly of models of this type. We review the early evolution and shell formation analyses and their generalizations to evolution in a density gradient. We also calculate the density and temperature that should be present in the

Crushing of interstellar gas clouds in supernova remnants - I. The role of thermal conduction and radiative losses

We model the hydrodynamic interaction of a shock wave of an evolved supernova remnant with a small interstellar gas cloud like the ones observed in the Cygnus loop and in the Vela SNR. We investigate the interplay between radiative cooling and thermal conduction during cloud evolution and their effect on the mass and energy exchange between the cloud and the surrounding medium. Through the study of two cases characterized by different Mach numbers of the primary shock (M = 30 and 50, corresponding to a post-shock temperature T 1.7 x 10 6 K and 4.7 x 10 6 K, respectively), we explore two very different physical regimes: for M = 30. the radiative losses dominate the evolution of the shocked cloud which fragments into cold, dense, and compact filaments surrounded by a hot corona which is ablated by the thermal conduction; instead, for M = 50, the thermal conduction dominates the evolution of the shocked cloud. which evaporates in a few dynamical time-scales. In both cases we find that the thermal conduction is very effective in suppressing the hydrodynamic instabilities that would develop at the cloud boundaries.