Speaker
Description
Scalar-tensor theories of gravity with a dynamical scalar field coupling non-minimally to matter via a conformal factor $A(\phi)$ pose computational challenges beyond standard general relativistic solvers. We present a fully numerical Python framework for constructing slowly rotating neutron star solutions in the massive scalar-tensor theory defined by the Einstein-frame coupling $\alpha(\phi) = \beta\phi$ with a dilaton potential, implemented within the Hartle slow-rotation expansion at first-order. The interior and exterior field equations for the metric potentials, scalar field, fluid pressure, and rotational drag function are cast as a coupled seven-dimensional first-order ODE system and integrated using an LSODA adaptive solver, which switches automatically between stiff and non-stiff regimes to handle the steep gradients introduced by the Yukawa-type scalar mass term. Surface-matching between domains is achieved through a Nelder-Mead shooting strategy that enforces asymptotic boundary conditions at spatial infinity, navigating the coexisting scalarized and general relativistic solution branches at fixed central energy density, with the APR equation of state interpolated via PCHIP in geometrized units throughout.
For each converged solution, the framework extracts the gravitational mass from the asymptotic metric gradient, the physical Jordan-frame radius via conformal rescaling, and the moment of inertia from the asymptotic behaviour of the rotational drag ODE. This yields complete mass-radius and moment-of-inertia–mass relations across the parameter space of coupling constant and scalar field mass. Our results demonstrate that the scalar mass plays a decisive role in shaping neutron star structure, with both relations deviating significantly from general relativity, establishing this framework as a reliable tool for probing the strong-field phenomenology of massive scalar-tensor gravity.