arXiv subjects

S. Rosswog

Publications and source records attributed to S. Rosswog.

67 records · Page 4Linked to original sources

arXiv2004

Accretion dynamics in neutron star black hole binaries

We perform three-dimensional, Newtonian hydrodynamic simulations with a nuclear equation of state to investigate the accretion dynamics in neutron star black hole systems. We find as a general result that non-spinning donor stars yield larger circularization radii than corotating donors. Therefore, the matter from a neutron star without spin will more likely settle into an accretion disk outside the Schwarzschild radius. With the used stiff equation of state we find it hard to form an accretion disk that is promising to launch a gamma-ray burst. In all relevant cases the core of the neutron star survives and keeps orbiting the black hole as a mini neutron star for the rest of the simulation time (up to several hundred dynamical neutron star times scales). The existence of this mini neutron star leaves a clear imprint on the gravitational wave signal which thus can be used to probe the physics at supra-nuclear densities.

astro-ph↗

arXiv2003

High Resolution Calculations of Merging Neutron Stars II: Neutrino Emission

The remnant resulting from the merger of two neutron stars produces neutrinos in copious amounts. In this paper we present the neutrino emission results obtained via Newtonian, high-resolution simulations of the coalescence event. These simulations use three-dimensional smoothed particle hydrodynamics together with a nuclear, temperature dependent equation of state and a multi-flavour neutrino leakage scheme. We present the details of our scheme, discuss the neutrino emission results from a neutron star coalescence and compare them to the core-collapse supernova case where neutrino emission has been studied for several decades. The average neutrino energies are similar to those in the supernova case, but contrary to the latter, the luminosities are dominated by electron-type antineutrinos which are produced in the hot, neutron-rich, thick disk of the merger remnant. The cooler parts of this disk contain substantial fractions of heavy nuclei, which, however, do not influence the overall neutrino emission results significantly. Our total neutrino luminosities from the merger event are considerably lower than those found in previous investigations. This has serious consequences for the ability to produce a gamma-ray burst via neutrino annihilation. The neutrinos are emitted preferentially along the initial binary rotation axis, an event seen ``pole-on'' would appear much brighter in neutrinos than a similar event seen ``edge-on''.

astro-ph↗

arXiv2002

Neutron Star Binaries as Central Engines of GRBs

We describe the results high resolution, hydrodynamic calculations of neutron star mergers. The model makes use of a new, nuclear equation of state, accounts for multi-flavour neutrino emission and solves the equations of hydrodynamics using the smoothed particle hydrodynamics method with more than $10^6$ particles. The merger leaves behind a strongly differentially rotating central object of $\sim 2.5$ M$_{\odot}$ together with a distribution of hot debris material. For the most realistic case of initial neutron star spins, no sign of a collapse to a black hole can be seen. We argue that the differential rotation stabilizes the central object for $\sim 10^2$ s and leads to superstrong magnetic fields. We find the neutrino emission from the hot debris around the freshly-formed, supermassive neutron star to be substantially lower than predicted previously. Therefore the annihilation of neutrino anti-neutrino pairs will have difficulties to power very energetic bursts ($\gg 10^{49}$ erg).

astro-ph↗

arXiv2002

Stellar Sources of the Interstellar Medium

With the exception of the Big Bang, responsible for 1,2H, 3,4He, and 7Li, stars act as sources for the composition of the interstellar medium. Cosmic rays are related to the latter and very probably due to acceleration of the mixed interstellar medium by shock waves from supernova remnants. Thus, the understanding of the abundance evolution in the interstellar medium and especially the enrichment of heavy elements, as a function of space and time, is essential. It reflects the history of star formation and the lifetimes of the diverse contributing stellar objects. Therefore, the understanding of the endpoints of stellar evolution is essential as well. These are mainly planetary nebulae and type II/Ib/Ic supernovae as evolutionary endpoints of single stars, but also events in binary systems can contribute, like e.g. supernovae of type Ia, novae and possibly X-ray bursts and neutron star or neutron star - black hole mergers. Despite many efforts, a full and self-consistent understanding of supernovae (the main contributors to nucleosynthesis in galaxies) is not existing, yet. Their fingerprints, however, seen either in spectra, lightcurves, radioactivities/decay gamma-rays or in galactic evolution, can help to constrain the composition of their ejecta and related model uncertainties.

astro-ph↗

arXiv2002

High Resolution Calculations of Merging Neutron Stars I: Model Description and Hydrodynamic Evolution

We present the results of 3D, high-resolution calculations of neutron star mergers. Using Newtonian gravity, but adding the forces from the emission of gravitational waves, we solve the hydrodynamics equations using the smoothed particle hydrodynamics method. The microscopic properties of the neutron star matter are described in terms of a new nuclear equation of state (EOS) based on the tables of Shen et al. (1998). The effects of neutrino emission of all flavours are accounted for. The new EOS is noticeably stiffer than the Lattimer-Swesty-EOS used in previous investigations. We find in general somewhat lower temperatures than previous investigations. This allows a substantial fraction of heavy nuclei to be present even in the inner parts of the torus that forms around the central object and yields important contributions to the neutrino opacity. The central object shows a rapid differential rotation with periods of $\sim$ 2ms, which we suspect to stabilize the central object on (at least) the simulation time scale against gravitational collapse. The differential rotation could wind up initial magnetic fields to $\sim 10^17$ G.

astro-ph↗

arXiv2002

The Supernova-GRB Connection

We discuss the possible connection between supernova explosions (SN) and gamma-ray bursters (GRB) from the perspective of our current understanding of SN physics. Recent evidence strongly suggests that the explosion mechanism of core collapse SN is intrinsically aspherical. Typically, a neutron star is formed. However, the observed properties of the expanding SN envelopes remnants make these objects very unlikely candidates for GRBs. Most candidates for a GRB/SN connection seem to require the prompt or delayed formation of a black hole. These include the collapse of very massive stars (e.g. hypernovae) and 'classical' SNe with a significant fallback of material over time scales of hours to days, resulting in the collapse of the neutron star to a black hole. We suggest the merger of a neutron star with a white dwarf as a subclass of thermonuclear SNe and a potential candidate for a SN/GRB connection.

astro-ph↗

arXiv2001

Towards a Macroscopic Modelling of the Complexity in Traffic Flow

We present a macroscopic traffic flow model that extends existing fluid-like models by an additional term containing the second derivative of the safe velocity. Two qualitatively different shapes of the safe velocity are explored: a conventional Fermi-type function and a function exhibiting a plateau at intermediate densities. The suggested model shows an extremely rich dynamical behaviour and shows many features found in real-world traffic data.

cond-mat.stat-mech↗

arXiv2001

Post Newtonian SPH

We introduce an adaptation of the well known Tree+SPH numerical scheme to Post Newtonian (PN) hydrodynamics and gravity. Our code solves the (0+1+2.5)PN equations. These equations include Newtonian hydrodynamics and gravity (0PN), the first order relativistic corrections to those (1PN) and the lowest order gravitational radiation terms (2.5PN). We test various aspects of our code using analytically solvable test problems. We then proceed to study the 1PN effects on binary neutron star coalescence by comparing calculations with and without the 1PN terms. We find that the effect of the 1PN terms is rather small. The largest effect arises with a stiff equation of state for which the maximum rest mass density increases. This could induce black hole formation. The gravitational wave luminosity is also affected.

astro-ph↗

arXiv2001

Element Synthesis in Stars

Except for 1H, 2H, 3He, 4He, and 7Li, originating from the Big Bang, all heavier elements are made in stellar evolution and stellar explosions. Nuclear physics, and in many cases nuclear structure far from stability, enters in a crucial way. Therefore, we examine in this review the role of nuclear physics in astrophysics in general and in particular how it affects stellar events and the resulting nucleosynthesis. Stellar modeling addresses four major aspects: 1. energy generation and nucleosynthesis, 2. energy transport via conduction, radiation or possibly convection, 3. hydrodynamics/hydrostatics, and finally 4. thermodynamic properties of the matter involved. Nuclear Physics enters via nuclear reaction cross sections and nuclear structure (affecting the composition changes and nuclear energy generation), neutrino-nucleon and neutrino-nucleus cross sections (affecting neutrino opacities and transport), and e.g. the equation of state at and beyond nuclear densities which creates a relation between the nuclear many-body problem and the hydrodynamic response like pressure and entropy. In the following we review these four topics by highlighting the role and impact of nuclear physics in each of these aspects of stellar modeling. The main emphasis is put on the connection to element synthesis.

astro-ph↗

arXiv2000

Nucleosynthesis Calculations for the Ejecta of Neutron Star Coalescences

We present the results of fully dynamical r-process network calculations for the ejecta of neutron star mergers (NSMs). The late stages of the inspiral and the final violent coalescence of a neutron star binary have been calculated in detail using a 3D hydrodynamics code (Newtonian gravity plus backreaction forces emerging from the emission of gravitational waves) and a realistic nuclear equation of state. The found trajectories for the ejecta serve as input for dynamical r-process calculations where all relevant nuclear reactions (including beta-decays depositing nuclear energy in the expanding material) are followed. We find that all the ejected material undergoes r-process. For an initial Ye close to 0.1 the abundance distributions reproduce very accurately the solar r-process pattern for nuclei with A above 130. For lighter nuclei strongly underabundant (as compared to solar) distributions are encountered. We show that this behaviour is consistent with the latest observations of very old, metal-poor stars, despite simplistic arguments that have recently been raised against the possibility of NSM as possible sources of Galactic r-process material.

astro-ph↗

arXiv2000

Merging neutron stars: asymmetric systems

Results of 3D, Newtonian hydrodynamic calculations of the last stages of the inspiral and the final coalescence of neutron star binary systems are described. The focus is on sligthly asymmetric systems with the asymmetry stemming either from different spins or different masses of the binary components. Implications for nucleosynthesis and gamma ray bursts are discussed. Test calculations for the used viscosity scheme are provided in the appendix.

astro-ph↗

arXiv1998

Mass ejection in neutron star mergers

We present the results of 3D Newtonian SPH simulations of the merger of a neutron star binary. The microscopic properties of matter are described by the physical equation of state of Lattimer and Swesty (LS-EOS). To test for the robustness of our results we check the sensitivity to the approximations of our model as well as to the binary system parameters. The main and new result is that for the realistic LS-EOS, depending on the initial spin, between 4e-3 and 4e-2 solar masses of material become unbound. If, as suggested, large parts of this matter consist of r-process nuclei, neutron star mergers could account for the whole observed r-process material in the Galaxy.

astro-ph↗

arXiv1998

Coalescing Neutron Stars: A Solution to the R-Process Problem?

Recent calculations place questions on the ability of supernovae to produce r-process nuclei in the correct amounts. We present results from 3D Newtonian SPH calculations of mergers of equal mass neutron star binaries. We find the amounts of ejected mass to be very promising for an explanation of the observed r-process abundances. Preliminary abundance calculations show that practically all the material is subject to r-process conditions. The calculated abundance patterns reproduce the basic features of the solar r-process abundances very well. Especially the peak around A= 195 is easily reproduced without any tuning of the initial entropies. Thus, eventually all the observed r-process material could be explained by mass ejection during neutron star mergers.

astro-ph↗