Modified gravity in the interior of population II stars

TL;DR

This study investigates how a specific class of modified gravity theories affects the internal structure and observable properties of a Population II star, providing potential constraints on the theory.

Contribution

It introduces a phenomenological model of a Population II star within a beyond-Horndeski gravity framework and analyzes its internal structure numerically.

Findings

Gravity effects can decrease luminosity and increase star radius.

Near the core, gravity effects are reversed, affecting stellar structure.

The model suggests a way to constrain modified gravity parameters.

Abstract

We study the effects of a beyond-Horndeski theory of modified gravity in the interior of a population II star. We consider a simple phenomenological model of a $1.1M_{\odot}$ star that has left the main sequence, has a thin Hydrogen burning shell with a partially degenerate isothermal core, surrounded by a radiative envelope having two regions of distinct opacities. Using suitable matching conditions at the two internal boundaries, a numerical analysis of the resulting stellar equations in modified gravity is carried out. While overall, gravity may be weakened, resulting in a decrease of the luminosity and an increase of the radius of the star, these effects are reversed near the core. It is suggested how the model, within its limitations, can yield a bound on the modified gravity parameter.