The Influence of Thermal Pressure on Equilibrium Models of Hypermassive Neutron Star Merger Remnants
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 (EOS), 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 EOS, 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 parameterized 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.
Additional Information© 2014 American Astronomical Society. Received 2013 June 17; accepted 2014 June 1; published 2014 June 27. We thank Eliot Quataert for inspiration and acknowledge helpful discussions with Lars Bildsten, Ursula C. T. Gamma, Jim Lattimer, Lee Lindblom, Sterl Phinney, Jocelyn Read, Yuichiro Sekiguchi, Masaru Shibata, Saul Teukolsky, Kip Thorne, and especially Aaron Zimmerman. Furthermore, we thank the anonymous referee for suggestions that improved this paper. This work was initiated at a Palomar Transient Factory Theory Network meeting at the Sky House, Los Osos, CA. C.D.O. thanks the Yukawa Institute for Theoretical Physics for hospitality during the long-term workshop Gravitational Waves and Numerical Relativity 2013 when this work was completed. This research is supported in part by NASA under the Astrophysics Theory grant No. NNX11AC37G, by the National Science Foundation under grant Nos. AST-1205732, PHY-1151197, AST-1212170, PHY-1068881, and PHY- 1068243, by the Alfred P. Sloan Foundation and by the Sherman Fairchild Foundation. The calculations underlying the results presented in this paper were performed on the Caltech compute cluster "Zwicky" (NSF MRI award No. PHY-0960291). The EOS tables, driver, and TOV solver routines used in this work are available for download at http://www.stellarcollapse.org. The solver for axisymmetric equilibrium configurations of rotating HMNSs is not open source, but a similar solver may be obtained from http://www.lorene.obspm.fr/. The figures in this paper were generated with the matplotlib library for Python (Hunter 2007). This work was supported by Grant-in-Aid for Scientific Research (25103510, 25105508, 24740163), by HPCI Strategic Program of Japanese MEXT. The simulations were performed on XC30 at CfCA of NAOJ and SR16000 at YITP of Kyoto University.
Published - 0004-637X_790_1_19.pdf
Submitted - 1306.4034v2.pdf