arXiv subjects

P. Chris Fragile

Publications and source records attributed to P. Chris Fragile.

76 records · Page 5Linked to original sources

arXiv2003

COSMOS: A Radiation-Chemo-Hydrodynamics Code for Astrophysical Problems

We have developed a new massively-parallel radiation-hydrodynamics code (Cosmos) for Newtonian and relativistic astrophysical problems that also includes radiative cooling, self-gravity, and non-equilibrium, multi-species chemistry. Several numerical methods are implemented for the hydrodynamics, including options for both internal and total energy conserving schemes. Radiation is treated using flux-limited diffusion. The chemistry incorporates 27 reactions, including both collisional and radiative processes for atomic hydrogen and helium gases, and molecular hydrogen chains. In this paper we discuss the equations and present results from test problems carried out to verify the robustness and accuracy of our code in the Newtonian regime. An earlier paper presented tests of the relativistic capabilities of Cosmos.

astro-ph↗

arXiv2003

Hydrodynamic Stability of Cosmological Quark-Hadron Phase Transitions

Recent results from linear perturbation theory suggest that first-order cosmological quark-hadron phase transitions occurring as deflagrations may be ``borderline'' unstable, and those occurring as detonations may give rise to growing modes behind the interface boundary. However, since nonlinear effects can play important roles in the development of perturbations, unstable behavior cannot be asserted entirely by linear analysis, and the uncertainty of these recent studies is compounded further by nonlinearities in the hydrodynamics and self-interaction fields. In this paper we investigate the growth of perturbations and the stability of quark-hadron phase transitions in the early Universe by solving numerically the fully nonlinear relativistic hydrodynamics equations coupled to a scalar field with a quartic self-interaction potential regulating the transitions. We consider single, perturbed, phase transitions propagating either by detonation or deflagration, as well as multiple phase and shock front interactions in 1+2 dimensional spacetimes.

gr-qc↗

arXiv2002

Non-Oscillatory Central Difference and Artificial Viscosity Schemes for Relativistic Hydrodynamics

High resolution, non-oscillatory, central difference (NOCD) numerical schemes are introduced as alternatives to more traditional artificial viscosity (AV) and Godunov methods for solving the fully general relativistic hydrodynamics equations. These new approaches provide the advantages of Godunov methods in capturing ultra-relativistic flows without the cost and complication of Riemann solvers, and the advantages of AV methods in their speed, ease of implementation, and general applicability without explicitly using artificial viscosity for shock capturing. Shock tube, wall shock, and dust accretion tests, all with adiabatic equations of state, are presented and compared against equivalent solutions from both AV and Godunov based codes. In the process we address the accuracy of time-explicit NOCD and AV methods over a wide range of Lorentz factors.

astro-ph↗

arXiv2001

Bardeen-Petterson Effect and Quasi-Periodic Oscillations in X-Ray Binaries

The Bardeen-Petterson effect around a rapidly-rotating compact object causes a tilted accretion disk to warp into the equatorial plane of the rotating body. Viscous forces cause the accretion flow to divide into two distinct regions - an inner aligned accretion disk and an outer tilted accretion disk. The transition between these two occurs at a characteristic radius that depends on the mass and angular momentum of the central object and possibly on the accretion rate through the disk. We propose that accreting material passing through the transition region may generate quasi-periodic brightness oscillations (QPOs) such as have been observed in a number of X-ray binaries. We show that this effect may be present in the black-hole X-ray binary GRO J1655-40. We also argue that the QPO frequency range predicted by this model is consistent with observed QPO frequencies in both black-hole and neutron-star low-mass X-ray binaries.

astro-ph↗