The Influence of Thermal Pressure on Equilibrium Models of Hypermassive Neutron Star Merger Remnants

Jeffrey D. Kaplan, Christian D. Ott, Evan P. O'Connor, Kenta Kiuchi, Luke Roberts and Matthew Duez
ApJ 790, 19, 2014, (ads)


The merger of two neutron stars leaves behind a rapidly spinning hypermassive object whose survival is believed to depend on the maximum mass supported by the nuclear equation of state, angular momentum redistribution by (magneto-)rotational instabilities, and spindown by gravitational waves. The high temperatures (~5-40 MeV) prevailing in the merger remnant may provide thermal pressure support that could increase its maximum mass and, thus, its life on a neutrino-cooling timescale. We investigate the role of thermal pressure support in hypermassive merger remnants by computing sequences of spherically-symmetric and axisymmetric uniformly and differentially rotating equilibrium solutions to the general-relativistic stellar structure equations. Using a set of finite-temperature nuclear equations of state, we find that hot maximum-mass critically spinning configurations generally do not support larger baryonic masses than their cold counterparts. However, subcritically spinning configurations with mean density of less than a few times nuclear saturation density yield a significantly thermally enhanced mass. Even without decreasing the maximum mass, cooling and other forms of energy loss can drive the remnant to an unstable state. We infer secular instability by identifying approximate energy turning points in equilibrium sequences of constant baryonic mass parametrized by maximum density. Energy loss carries the remnant along the direction of decreasing gravitational mass and higher density until instability triggers collapse. Since configurations with more thermal pressure support are less compact and thus begin their evolution at a lower maximum density, they remain stable for longer periods after merger.


Equation of State Tables

As discussed in detail in the paper, we use barotropic (P=P[rho]) version of a set of fully temperature dependent equation of state tables. The original tables used can be obtained from

We will soon provide the code that we use to generate the P[rho] tables. For the meantime, we provide isothermal and cXpX (see Kaplan et al. 2013) temperature prescriptions and neutrino-full beta equilibrium. These are the tables used for the TOV results presented in the paper.

Download EOS tables (~4 MB)


Tolman-Oppenheimer-Volkhoff Solver

We provide a simple TOV solver that can handle the above P[rho] equation of state files: Download

The code is quite self explanatory and should be easy to run. A Makefile is included and set up to work with gfortran. Provide with an EOS table (as a command line argument), the code will solve (and output key parameters) for a sequence of TOV solutions parametrized by central baryon density.

Should you have any problem with the solver, please contact Christian Ott (cott #at#