arXiv subjects

Tomasz Plewa

Publications and source records attributed to Tomasz Plewa.

23 records · Page 2Linked to original sources

arXiv1999

Reconstructing Cosmic Peculiar Velocities from the Mildly Nonlinear Density Field

We present a numerical study of the cosmic density vs. velocity divergence relation (DVDR) in the mildly non-linear regime. We approximate the dark matter as a non-relativistic pressureless fluid, and solve its equations of motion on a grid fixed in comoving coordinates. Unlike N-body schemes, this method yields directly the volume-averaged velocity field. The results of our simulations are compared with the predictions of the third-order perturbation theory (3PT) for the DVDR. We investigate both the mean `forward' relation (density in terms of velocity divergence) and the mean `inverse' relation (velocity divergence in terms of density), with emphasis on the latter. On scales larger than about 20 megaparsecs, our code recovers the predictions of 3PT remarkably well, significantly better than recent N-body simulations. On scales of a few megaparsecs, the DVDR predicted by 3PT differs slightly from the simulated one. In particular, approximating the inverse DVDR by a third-order polynomial turns out to be a poor fit. We propose a simple analytical description of the inverse relation, which works well for mildly non-linear scales.

astro-ph↗

arXiv1998

The Consistent Multi-fluid Advection method

Simple modifications for higher-order Godunov-type difference schemes are presented which allow for accurate advection of multi-fluid flows in hydrodynamic simulations. The constraint that the sum of all mass fractions has to be equal to one in every computational zone throughout the simulation is fulfilled by renormalizing the mass fractions during the advection step. The proposed modification is appropriate for any difference scheme written in conservation form. Unlike other commonly used methods it does not violate the conservative character of the advection method. A new steepening mechanism, which is based on modification of interpolation profiles, is used to reduce numerical diffusion across composition discontinuities. Additional procedures are described, which are necessary to enforce monotonicity. Several numerical experiments are presented which demonstrate the capability of our Consistent Multi-fluid Advection (CMA) method in case of smooth and discontinuous distributions of fluid phases and under different hydrodynamic conditions. It is shown that due to the reduced diffusivity of the proposed scheme the abundance of some heavy elements obtained from hydrodynamic simulations of type II supernova explosions can change by a factor of a few in the most extreme cases.

astro-ph↗

arXiv1998

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

(shortened version) 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_sun, 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. 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 demonstrate a simple technique for estimating the optical emission expected. These tools are employed to choose parameters of models which we then explore with our 1d and 2d hydrocodes, providing, respectively, the detailed x-ray spectra and dynamical characteristics.

astro-ph↗

arXiv1996

Modern Numerical Hydrodynamics and the Evolution of Dense Interstellar Medium

We discuss some typical problems related to numerical hydrodynamics of a dense interstellar medium. A newly developed hydrodynamical code based on adaptive mesh refinement technique is presented and applied to simulate the evolution of a supernova remnant in a high-density medium. Advantages of this new approach in comparison to commonly used hydrodynamical methods are briefly discussed.

astro-ph↗

arXiv1994

Fast Radiative Shocks in Dense Media. III. Properties of the Emission

Evolution of fast, radiative shocks in high density medium is presented. Ionizing spectra and approximate broad band light curves of the shocked gas are calculated. Emergent shock spectra, as seen by a distant observer, are obtained from photoionization models. The emergent spectra have a power-law shape $F_ν\proptoν^{-α}$ with mean spectral index $α\sim0.6-1.0$ in the energy range $0.01-10$ keV, and have a high-energy cutoff corresponding to the original shock velocity. It is shown that the models exhibit promising features that may account for some photometric and spectral properties of Active Galactic Nuclei.

astro-ph↗