AbstractWe study solutions of the stellar structure equations for spherically symmetric objects in modified theories of gravity, where the Einstein-Hilbert Lagrangian is replaced by $$f(R)=R+\alpha R^2$$
f
(
R
)
=
R
+
α
R
2
and $$f(R,Q)=R+\alpha R^2+\beta Q$$
f
(
R
,
Q
)
=
R
+
α
R
2
+
β
Q
, with R being the Ricci scalar curvature, $$Q=R_{\mu \nu }R^{\mu \nu }$$
Q
=
R
μ
ν
R
μ
ν
and $$R_{\mu \nu }$$
R
μ
ν
the Ricci tensor. We work in the Palatini formalism, where the metric and the connection are assumed to be independent dynamical variables. We focus on stellar solutions in the mass-radius region associated to neutron stars. We illustrate the potential impact of the $$R^2$$
R
2
and Q terms by studying a range of viable values of $$\alpha $$
α
and $$\beta $$
β
. Similarly, we use different equations of state (SLy, FPS, HS(DD2) and HS(TMA)) as a simple way to account for the equation of state uncertainty. Our results show that for certain combinations of the $$\alpha $$
α
and $$\beta $$
β
parameters and equation of state, the effect of modifications of general relativity on the properties of stars is sizeable. Therefore, with increasing accuracy in the determination of the equation of state for neutron stars, astrophysical observations may serve as discriminators of modifications of General Relativity.
Invariant quantities of scalar–tensor theories for stellar structure
