Stability of hypermassive neutron stars with realistic rotation and entropy profiles

Creators: Muhammed, Nishad¹; Duez, Matthew D.¹; Chawhan, Pavan¹; Ghadiri, Noora²; Buchman, Luisa T.¹; Foucart, Francois³; Cheong, Patrick Chi-Kit³; Kidder, Lawrence E.⁴; Pfeiffer, Harald P.⁵; Scheel, Mark A.⁶

1. Washington State University
2. University of Illinois Urbana-Champaign
3. University of New Hampshire
4. Cornell University
5. Max Planck Institute for Gravitational Physics
6. California Institute of Technology

Abstract

Binary neutron star mergers produce massive, hot, rapidly differentially rotating neutron star remnants; electromagnetic and gravitational wave signals associated with the subsequent evolution depend on the stability of these remnants. Stability of relativistic stars has previously been studied for uniform rotation and for a class of differential rotation with monotonic angular velocity profiles. Stability of those equilibria to axisymmetric perturbations was found to respect a turning point criterion: along a constant angular momentum sequence, the onset of unstable stars is found at maximum density less than but close to the density of maximum mass. In this paper, we test this turning point criterion for nonmonotonic angular velocity profiles and nonisentropic entropy profiles, both chosen to more realistically model postmerger equilibria. Stability is assessed by evolving perturbed equilibria in 2D using the spectral einstein Code. We present tests of the code’s new capability for axisymmetric metric evolution. We confirm the turning point theorem and determine the region of our rotation law parameter space that provides highest maximum mass for a given angular momentum.

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Acknowledgement

M. D. gratefully acknowledges support from the NSF through Grant No. PHY-2110287 and through REU Grant No. PHY-2050886. M. D. and F. F. gratefully acknowledge support from NASA through Grant No. 80NSSC22K0719. F. F. gratefully acknowledge support from the Department of Energy, Office of Science, Office of Nuclear Physics, under Contract No. DE-AC02-05CH11231 and from the NSF through Grant No. AST-2107932. M. S. acknowledges funding from the Sherman Fairchild Foundation and by NSF Grants No. PHY-1708212, No. PHY-1708213, and No. OAC-1931266 at Caltech. L. K. acknowledges funding from the Sherman Fairchild Foundation and by NSF Grants No. PHY-1912081, No. PHY-2207342, and No. OAC-1931280 at Cornell. Computations for this manuscript were performed on the Wheeler cluster at Caltech, supported by the Sherman Fairchild Foundation. P. C.-K. C. acknowledge support from NSF Grant No. PHY-2020275 (Network for Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS)).

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