Equilibrium, radial stability and non-adiabatic gravitational collapse of anisotropic neutron stars
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Regular Article - Theoretical Physics
Equilibrium, radial stability and non-adiabatic gravitational collapse of anisotropic neutron stars Juan M. Z. Pretela Instituto de Física, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ CEP 21941-972, Brazil
Received: 15 June 2020 / Accepted: 29 July 2020 © The Author(s) 2020
Abstract In this work we construct families of anisotropic neutron stars for an equation of state compatible with the constraints of the gravitational-wave event GW170817 and for four anisotropy ansatze. Such stars are subjected to a radial perturbation in order to study their stability against radial oscillations and we develop a dynamical model to describe the non-adiabatic gravitational collapse of the unstable anisotropic configurations whose ultimate fate is the formation of a black hole. We find that the standard criterion for radial stability d M/dρc > 0 is not always compatible with the calculation of the oscillation frequencies for some anisotropy ansatze, and each anisotropy parameter is constrained taking into account the recent restriction of maximum mass of neutron stars. We further generalize the TOV equations within a non-adiabatic context and we investigate the dynamical behaviour of the equation of state, heat flux, anisotropy factor and mass function as an unstable anisotropic star collapses. After obtaining the evolution equations we recover, as a static limit, the background equations.
1 Introduction The most common matter-energy distribution for modeling the internal structure of compact stars is an isotropic perfect fluid. Nevertheless, there are strong arguments suggesting that nuclear matter at very high densities and pressures could naturally be described by an anisotropic fluid, that is, when the radial and tangential components of the pressure are not equal. As a matter of fact, the anisotropy could be generated by the presence of strong magnetic fields [1–4], solid cores [5, 6], superfluidity [7,8], pion condensed phase configurations in neutron stars [9,10], etc. Furthermore, it is possible to obtain an anisotropic perfect fluid by combining the energymomentum tensors of two isotropic perfect fluids [11–13]. a e-mail:
As widely reported in the literature [14–25], the presence of anisotropy affects a number of important physical properties of compact stars such as the mass-radius relation, compactness, surface redshift, moment of inertia as well as the scalarization in scalar-tensor theories [26]. Indeed, the equation of state (EoS) plays a fundamental role in determining the internal structure of such stars and, consequently, in imposing stability limits. Therefore, it is important to carry out a stability analysis of anisotropic neutron stars taking into account the LIGO-Virgo constraints on the EoS for nuclear matter as a result of observation of the event GW170817 – the first direct detection of gravitational waves from the coalescence of a neutron star binary system [27,28]. In that respect, we are interested in considering a realistic EoS, which is compat
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