# Relativistic stars in degenerate higher-order scalar-tensor theories after GW170817

## TL;DR

This paper investigates how certain scalar-tensor theories, compatible with gravitational wave speed constraints, alter the structure and properties of relativistic stars, especially their mass-radius relation and maximum density.

## Contribution

It demonstrates that degenerate higher-order scalar-tensor theories can significantly modify stellar structure without conflicting with GW170817 constraints.

## Key findings

- Exterior metric remains Schwarzschild if Horndeski terms are ignored
- Partial breaking of Vainshtein screening affects stellar interior
- High-density stars show significant mass-radius relation deviations

## Abstract

We study relativistic stars in degenerate higher-order scalar-tensor theories that evade the constraint on the speed of gravitational waves imposed by GW170817. It is shown that the exterior metric is given by the usual Schwarzschild solution if the lower order Horndeski terms are ignored in the Lagrangian and a shift symmetry is assumed. However, this class of theories exhibits partial breaking of Vainshtein screening in the stellar interior and thus modifies the structure of a star. Employing a simple concrete model, we show that for high-density stars the mass-radius relation is altered significantly even if the parameters are chosen so that only a tiny correction is expected in the Newtonian regime. We also find that, depending on the parameters, there is a maximum central density above which solutions cease to exist.