Horndeski stars

Abstract

We establish the existence of time-dependent solitons in a modified gravity framework, which is defined by the low energy limit of theories with a weakly broken galileon symmetry and a mass term. These are regular vacuum configurations of finite energy characterized by a single continuous parameter representing the amplitude of the scalar degree of freedom at the origin. When the central field amplitude is small the objects are indistinguishable from boson stars. In contrast, increasing the central value of the amplitude triggers the effect of higher derivative operators in the effective theory, leading to departures from the previous solutions, until the theory becomes strongly coupled and model-dependent. The higher order operators are part of the (beyond) Horndeski theory, hence the name of the compact objects. Moreover, a remnant of the galileon non-renormalization theorem guarantees that the existence and properties of these solutions are not affected by quantum corrections. Finally, we discuss the linear stability under small radial perturbations, the mass-radius relation, the compactness, the appearance of innermost stable circular orbits and photon spheres, and some astrophysical signatures (accretion disks, gravitational radiation and lensing) that may be relevant to falsify the model.

Gravitational decoupling of generalized Horndeski hybrid stars

“…Besides, for fixed Γ and α, for c 4 > 0 [c 4 < 0] both the scalar field and the fermionic fluid pressure are smoother [sharper] along the radial coordinate, when compared to the gravitational decoupled Einstein-Klein-Gordon system, which corresponds to c 4 = 0. Hence, one can conclude that the gravitational decoupling setup emulates standard results in extended Horndeski scalar-tensor gravity [73,84], also corroborating to the conjecture that those negative…”

“…Figs.3 and 4regard gravitational decoupled hybrid stars with a fermionic core. The type of core in hybrid stars is encoded by the choice of the parameters Λ, σ 0 , and p 0[84,87]. In all plots that follow, the polytropic constant K = 10 2 m −2 M −2 p is employed, corroborating to phenomenological data.…”

“…In all plots that follow, the polytropic constant K = 10 2 m −2 M −2 p is employed, corroborating to phenomenological data. The involved parameters were taken in the parameter space derived in Refs [84,87],. in such a way that fermionic matter emulates typical astrophysical neutron stellar configurations, with central density ρ 0 = lim r→0 ρ(r) ∼ 10 12 ρ and central pressure p 0 ∼ 10 18 p .…”

Extreme $\ell$-boson stars

“…These orbits are stable wherever L(r) grows with r, whereas they are unstable otherwise [50]. For regular configurations light rings can appear only in pairs, one of them being stable and the other unstable.…”

“…where the sign of the second derivative of the lapse function evaluated at the light ring radius determines the stability of the orbit: it is stable if α ′′ is negative and unstable otherwise [50]. In the timelike case the energy and total angular momentum per unit rest mass of a particle in circular motion at radius r must satisfy…”