Astron. Astrophys. 360, 171-184 (2000)

Merging neutron stars: asymmetric systems

S. Rosswog ¹, M.B. Davies ², F.-K. Thielemann ³ and T. Piran <sup>4

1</sup> Center for parallel computing (ZPR/ZAIK), Universität zu Köln, Germany
² Department Physics and Astronomy, University of Leicester LEI 7RH, UK
³ Departement für Physik und Astronomie, Universität Basel, Switzerland
⁴ Racah Institute for Physics, Hebrew University, Jerusalem, Israel

Received 2 June 1999 / Accepted 17 May 2000

Abstract

We present the results of 3D, Newtonian hydrodynamic calculations of the last stages of the inspiral and the final coalescence of neutron star binary systems. Our focus is on slightly asymmetric systems, where the asymmetry stems from either different masses (1.3 and 1.4 ) or spins of both components. Almost immediately after contact a fast rotating, very massive central object forms. All calculations exhibit baryonic masses above 2.3 , thus based on our calculations it is not possible to decide on the fate of the central core of the merged configuration. It might collapse immediately to a black hole, but also the creation of a supermassive neutron star with cannot firmly be excluded. Depending on the asymmetry of the system the central object receives a kick of several hundred kilometers per second. Different spins of both components do not jeopardize the formation of (to within numerical resolution) baryon free funnels above the poles of the central objects. In the case of different masses the less massive components get disrupted and engulf the more massive companions that stay rather unaffected by the collision. The amount of ejected material is in a similar range as for symmetric systems and could contribute substantially to the enrichment of the Galaxy with heavy r-process elements. Test calculations indicate that the amount of ejected material is basically determined by the high density behaviour of the nuclear equation of state.

Test calculations for the hybrid artificial viscosity scheme that is used for this work are given in the appendix.

Key words: hydrodynamics stars: binaries: close stars: neutron gamma rays: bursts

Send offprint requests to: S. Rosswog (rosswog@zpr.uni-koeln.de)

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Contents

• 1. Introduction
• 2. The model
  • 2.1. The numerical method
  • 2.2. Gravitational radiation backreaction
  • 2.3. Artificial viscosity
  • 2.4. The nuclear equation of state
  • 2.5. Masses
  • 2.6. Stellar spins
  • 2.7. Tidal deformation
• 3. Results
  • 3.1. Morphology
  • 3.2. Mass distribution
  • 3.3. Kick velocities
  • 3.4. Baryonic pollution/GRBs
  • 3.5. Temperatures
  • 3.6. Ejecta
• 4. Summary and discussion
• Acknowledgements
• Appendices
  • A: artificial viscosity
• References

© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000
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