Speaker
Description
In this work, we model slowly rotating neutron stars in massive scalar-tensor gravity, focusing on the massive Damour-Esposito-Farese model. 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. In this theory, matter is coupled to an additional scalar field, and sufficiently compact stars can undergo spontaneous scalarization, where the scalar field grows inside and around the star and changes its macroscopic properties. It is implemented within the Hartle slow-rotation expansion at first-order.
We solve the stellar structure equations using the APR equation of state, interpolated via PCHIP in geometrized units throughout and a shooting-based numerical scheme using the Nelder-Mead optimizer. Two numerical improvements are introduced. First, the outer boundary condition is chosen to match the expected Yukawa decay of a massive scalar field, leading to a Robin boundary condition that strongly reduces sensitivity to the finite outer boundary. Second, the stellar surface is recalibrated using energy-density gradients and sequence-based smoothing, improving radius-dependent observables while preserving genuine physical peaks.
Using this framework, we compute mass-radius relations, central-density sequences, moments of inertia, and other stellar profiles for different scalar coupling strengths and scalar masses. We find that stronger coupling and lighter scalar fields produce the largest deviations from general relativity, including higher masses and modified radii. These results show that accurate boundary treatment and surface identification are essential for robust predictions of scalarized neutron stars and for connecting scalar-tensor gravity to future astrophysical observations.