Non-relativistic stellar structure in higher-curvature gravity: systematic construction of solutions to the modified Lane-Emden equations

Abstract: We study the structure of static spherical stars made up of a non-relativistic polytropic fluid in linearized higher-curvature theories of gravity (HCG). We first formulate the modified Lane-Emden (LE) equation for the stellar profile function in a gauge-invariant manner, finding it boils down to a sixth order differential equation in the generic case of HCG, while it reduces to a fourth order equation in two special cases, reflecting the number of additional massive gravitons arising in each theory. Moreover, the existence of massive gravitons renders the nature of the boundary-value problem unlike the standard LE: some of the boundary conditions can no longer be formulated in terms of physical conditions at the stellar center alone, but some demands at the stellar surface necessarily come into play. We present a practical scheme for constructing solutions to such a problem and demonstrate how it works in the cases of the polytropic index n = 0 and 1, where analytical solutions to the modified LE equations exist. As physical outcomes, we clarify how the stellar radius, mass, and Yukawa charges depend on the theory parameters and how these observables are mutually related. Reasonable upper bounds on the Weyl-squared correction are obtained.