Neutron star scalarization with Gauss-Bonnet and Ricci scalar couplings

Abstract

Spontaneous scalarization of neutron stars has been extensively studied in the Damour and Esposito-Farèse model, in which a scalar field couples to the Ricci scalar or, equivalently, to the trace of the energy-momentum tensor. However, scalarization of both black holes and neutron stars may also be triggered by a coupling of the scalar field to the Gauss-Bonnet invariant. The case of the Gauss-Bonnet coupling has also received a lot of attention lately, but the synergy of the Ricci and Gauss-Bonnet couplings has been overlooked for neutron stars. Here, we show that combining both couplings has interesting effects on the properties of scalarized neutron stars, such as affecting their domain of existence or the amount of scalar charge they carry.

Constraining modified gravity theories with scalar fields using black-hole images

“…In general we can distinguish between three regions: (I) β 1 solutions are unstable, (II) 1 β 1.2 the solution curves have both a stable and an unstable part (effectively yielding one stable and one unstable solution for any M), and (III) β > β crit ≈ 1.2 when all solutions are stable. Finally, values of β close to one achieve scalarization suppression for neutron stars [124] and avoid significantly influencing the formation of Large Scale Structures.…”

“…Here, we consider the minimal model associated with spontaneous scalarization identified in [120] and explored in [119,[122][123][124][125], where the coupling functions are defined as…”

“…This subclass of scalarization models has particular interest as it addresses a number of issues traditionally associated with scalarization. Specifically: (i) it suppresses neutron-star scalarization leading to avoidance of binary pulsar constraints [124], (ii) it allows for a late time cosmological attractor to GR [123], (iii) it yields stable scalarized black-hole solutions [119,125], and (iv) it improves the hyperbolicity of the formulation [119]. Considering the various benefits of this sRGB synergy, here we try to test its implications to black-hole shadows.…”

Astrophysical constraints on compact objects in 4D Einstein-Gauss-Bonnet gravity

“…The cubic [96] and quartic [97,98] terms have been investigated in this respect. The quintic sector has also received a lot of attention, especially in the form of a coupling to the Gauss-Bonnet scalar, in the context of scalarization [77,99]. Interesting effects have been pointed out in the extension of Horndeski theory known as degenerate higher-order scalar-tensor (DHOST) theories.…”

Neutron stars in scalar torsion theories with non minimal coupling

“…In addition to their conventional properties, NSs exhibit novel features in certain theories. These include the emergence of non-trivial scalar hairs, initially discovered by Damour and Esposito-Farèse [38] and extensively discussed in subsequent works [39][40][41][42][43], the manifestation of spontaneous vectorization in Proca theories [44], and the potential existence of new types JCAP05(2024)057 of NSs with topological charges [45]. Moreover, numerous studies have explored universal relations between the normalized moments of inertia (I) and the stellar compactness (C) of the star [46][47][48].…”