Stability of neutron stars in Horndeski theories with Gauss-Bonnet couplings

Abstract

In Horndeski theories containing a scalar coupling with the Gauss-Bonnet (GB) curvature invariant ${R}_{\mathrm{GB}}^{2}$, we study the existence and linear stability of neutron star (NS) solutions on a static and spherically symmetric background. For a scalar-GB coupling of the form $\ensuremath{\alpha}\ensuremath{\xi}(\ensuremath{\phi}){R}_{\mathrm{GB}}^{2}$, where $\ensuremath{\xi}$ is a function of the scalar field $\ensuremath{\phi}$, the existence of linearly stable stars with a nontrivial scalar profile without instabilities puts an upper bound on the strength of the dimensionless coupling constant $|\ensuremath{\alpha}|$. To realize maximum masses of NSs for a linear (or dilatonic) GB coupling ${\ensuremath{\alpha}}_{\mathrm{GB}}\ensuremath{\phi}{R}_{\mathrm{GB}}^{2}$ with typical nuclear equations of state, we obtain the theoretical upper limit $\sqrt{|{\ensuremath{\alpha}}_{\mathrm{GB}}|}<0.7\text{ }\text{ }\mathrm{km}$. This is tighter than those obtained by the observations of gravitational waves emitted from binaries containing NSs. We also incorporate cubic-order scalar derivative interactions, quartic derivative couplings with nonminimal couplings to a Ricci scalar besides the scalar-GB coupling, and show that NS solutions with a nontrivial scalar profile satisfying all the linear stability conditions are present for certain ranges of the coupling constants. In regularized four-dimensional Einstein-GB gravity obtained from a Kaluza-Klein reduction with an appropriate rescaling of the GB coupling constant, we find that NSs in this theory suffer from a strong coupling problem as well as Laplacian instability of even-parity perturbations. We also study NS solutions with a nontrivial scalar profile in power-law $F({R}_{\mathrm{GB}}^{2})$ models, and show that they are pathological in the interior of stars and plagued by ghost instability together with the asymptotic strong coupling problem in the exterior of stars.