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Hypermassive Stars:  Their Structure and Demise

Dr. Matthew Duez
Department of Physics and Astronomy
Washington State University

The merger of two neutron stars can give rise to an unusual type of object:  a hypermassive neutron star.  This is a configuration of nuclear matter that can persist many dynamical timescales although being significantly more massive than the maximum mass of an ordinary neutron star.  An equilibrium of this type is held up against collapse to a black hole with the help of thermal pressure and strong differential rotation.  Numerical relativity is the main tool used to probe the structure and stability of hypermassive stars, both for the study binary neutron star mergers and to investigate this new class of self-gravitating equilibria.  Numerical simulations are also used to probe the evolution of these objects, because although they are (sometimes) dynamically stable, on longer, secular timescales (which for hypermassive neutron stars may still be less than a second), they are altered by effects such as turbulent momentum transport and radiative cooling, which sap away the star's supports against collapse.  The object formed by binary neutron star merger event GW170817, for example, is thought to have collapsed to a black hole fairly quickly.  In this talk, I will summarize what relativists have learned about hypermassive stars, using order of magnitude estimates when possible, citing numerical results when necessary, and explain techniques being developed to model the secular evolution of these short-lived objects.