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Register a new userNonlinear evolution and non-uniqueness of scalarized neutron stars
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It was recently shown, that in a class of tensor-multi-scalar theories of gravity with a nontrivial target space metric, there exist scalarized neutron star solutions. An important property of these compact objects is that the scalar charge is zero and therefore, the binary pulsar experiments can not impose constraints based on the absence of scalar dipole radiation. Moreover, the structure of the solutions is very complicated. For a fixed central energy density up to three neutron star solutions can exist -- one general relativistic and two scalarized, that is quite different from the scalarization in other alternative theories of gravity. In the present paper we address the stability of these solutions using two independent approaches -- solving the linearized radial perturbation equations and performing nonlinear simulations in spherical symmetry. The results show that the change of stability occurs at the maximum mass models and all solutions before that point are stable. This leads to the interesting consequence that there exists a stable part of the scalarized branch close to the bifurcation point where the mass of the star increases with the decrease of the central energy density.
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In a certain class of scalar-Gauss-Bonnet gravity, the black holes and the neutron stars can undergo spontaneous scalarization - a strong gravity phase transition triggered by a tachyonic instability due to the non-minimal coupling between the scalar field and the spacetime curvature. Studies of this phenomenon have so far been restricted mainly to the study of the tachyonic instability and stationary scalarized black holes and neutron stars. Up to date there has been proposed no realistic physical mechanism for the formation of isolated scalarized black holes and neutron stars. We study for the first time the stellar core collapse to a black hole and a neutron star in scalar-Gauss-Bonnet theories allowing for a spontaneous scalarization. We show that the core collapse can produce scalarized black holes and scalarized neutron stars starting with a non-scalarized progenitor star.
Scalar-tensor theories are well studied extensions of general relativity that offer deviations which are yet within observational boundaries. We present the time evolution equations governing the perturbations of a nonrotating scalarized neutron star, including a dynamic spacetime as well as scalar field within the framework of such scalar-tensor theories. We employ a theory that allows for a massive scalar field or a self-interaction term and we study the impact of those parameters on the non-axisymmetric $f$-mode. The time evolution approach allows for a comparatively simple implementation of the boundary conditions. We find that the $f$-mode frequency is no longer a simple function of the stars average density when a scalar field is present. We also evaluate the accuracy of different variants of the Cowling approximation commonly used in previous studies of neutron star oscillation modes in alternative theories of gravity and demonstrate that it can give us not only qualitatively correct results, but in some cases also good quantitative estimates of the oscillations frequencies.
In the present paper we show the existence of a fully nonlinear dynamical mechanism for the formation of scalarized black holes which is different from the spontaneous scalarization. We consider a class of scalar-Gauss-Bonnet gravity theories within which no tachyonic instability can occur. Although the Schwarzschild black holes are linearly stable against scalar perturbations, we show dynamically that for certain choices of the coupling function they are unstable against nonlinear scalar perturbations. This nonlinear instability leads to the formation of new black holes with scalar hair. The fully nonlinear and self-consistent study of the equilibrium black holes reveals that the spectrum of solutions is more complicated possessing additional branches with scalar field that turn out to be unstable, though. The formation of scalar hair of the Schwarzschild black hole will always happen with a jump because the stable scalarized branch is not continuously connected to Schwarzschild one.
We compute the internal modes of a non-spinning neutron star and its tidal metric perturbation in general relativity, and determine the effect of relativistic corrections to the modes on mode coupling and the criterion for instability. Claims have been made that a new hydrodynamic instability can occur in a neutron star in a binary neutron star system triggered by the nonlinear coupling of the companions tidal field to pairs of p-modes and g-modes in it as the binary inspirals toward merger. This PG instability may be significant since it can influence the binarys inspiral phase by extracting orbital energy, thereby potentially causing large deviations in their gravitational waveforms from those predicted by theoretical models that do not account for it. This can result in incorrect parameter estimation, at best, or mergers going undetected, at worst, owing to the use of deficient waveform models. On the other hand, better modeling of this instability and its effect on binary orbits can unravel a new phenomenon and shed light on stellar instabilities, via gravitational wave observations. So far, all mode-tide coupling instability studies have been formulated in Newtonian perturbation theory. Neutron stars are compact objects, so relativistic corrections might be important. We present and test a new code to calculate the relativistic eigenmodes of nonrotating relativistic stars. We use these relativistic tide and neutron star eigenmodes to compute the mode-tide coupling strength (MTCS) for a few selected equations of state. The MTCS thus calculated can be at most tens of percent different from its purely Newtonian value, but we confirm the dependencies on orbital separation and equation of state found by Newtonian calculations. For some equations of state, the MTCS is very sensitive to the neutron star crust region, demonstrating the importance of treating this region accurately.
In this work we analyze hydrostatic equilibrium configurations of neutron stars in a non-minimal geometry-matter coupling (GMC) theory of gravity. We begin with the derivation of the hydrostatic equilibrium equations for the $f(R,L) $ gravity theory, where $R$ and $L$ are the Ricci scalar and Lagrangian of matter, respectively. We assume $f(R,L)=R/2+[1+sigma R]L$, with $sigma$ constant. To describe matter inside neutron stars we assume the polytropic equation of state $p=K rho^{gamma}$, with $K$ and $gamma = 5/3 $ being constants. We show that in this theory it is possible to reach the mass of massive pulsars such as PSR J2215+5135. As a feature of the GMC theory, very compact neutron stars with radius $sim8$km and $Msim 2.6M_odot$ are stable, thus surpassing the Buchdal and Schwarzschild radius limits. Moreover, the referred stellar diameter is obtained within the range of observational data.
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