Heavy elements from black hole-neutron star mergers

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Created on Thursday, 28 January 2016 19:20

Matter flung out into space during black hole-neutron star mergers may well be one of the major contributors to the "r-process" heavy-elements in the universe such as gold and lead.  To test this idea, we calculated the nuclear reactions taking place in the ejected matter from our black hole -neutron star mergers using the "SkyNet" nucleosynthesis code written by SXS member Jonas Lippuner.  Our first studies found that it is very easy for our ejecta to produce the high-mass r-process elements (in abundance ratios not terribly different from what's in the sun), but very little of the low-mass r-process elements are made, meaning they would have to come from a different source.  We then found that this underproduction can be ameliorated (but not removed) by the effect of neutrinos given off by the merger remnant being absorbed by the outgoing matter, which changes its composition.  Read the paper here (http://arxiv.org/abs/1601.07942).

SXS studies binary neutron star binary mergers

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Created on Wednesday, 21 October 2015 19:24

The inspiral and merger of binaries composed of two neutron stars are excellent gravitational wave sources and strong candidate explanations for short duration gamma ray bursts.  In a series of recent papers, SXS researchers have joined the effort to model these violent events.  Nick Tacik and collaborators have been studying the usually-overlooked effect of the spins of the two neutron stars on their final orbits of inspiral.  Read the paper here (http://arxiv.org/abs/1508.06986).  Roland Haas, working with many on the SXS team, has carried our first binary neutron star simulation all the way through the collision of the two stars and the collapse of the giant merged neutron star into a black hole.  A paper on this simulation is still in the works, but it's already been used to test a recipe for generating binary neutron star waveforms without full numerical simulations of these events.  The paper, by Barkett et al, is here (http://arxiv.org/abs/1509.05782) The idea is that binary black hole simulations are cheaper than binary neutron star simulations, and one can roughly convert the former into the latter by adding an analytic approximation of the neutron star tidal effects. Lastly, SXS researcher Francois Foucart has carried out merger simulations that study in detail what happens right after two neutron stars merge into a single more-massive neutron star.  These post-merger neutron stars ring and give off possibly-detectable gravitational waves.  Studying the post-merger mess required us to incorporate realistic nuclear physics and neutrino emission effects into simulations.  Read the paper here (http://arxiv.org/abs/1510.06398).

Survey of aligned-spin binary black holes

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Created on Wednesday, 20 January 2016 19:37

The SXS collaboration has produced a catalog of about 90 simulations of binary black holes with spins aligned with the orbital angular momentum.  We sample systems with both spins co-rotating, one co-rotating and one counter-rotating, or both counter-rotating.  We compare these simulations with several waveform models in use by LIGO, and find generally excellent agreement.  The papers can be accessed arXiv:1512.06800  and arXiv:1601.05396.