Massive boson stars: Stability and GW emission in head-on mergers

Abstract

We investigate quartically self-interacting massive boson stars by constructing equilibrium sequences and performing dynamical evolutions. The mass curve $M(|ϕ_c|)$ along these sequences develops multiple extrema, yet stability changes only at the first maximum; configurations beyond it become highly compact and collapse under numerically induced perturbations, with near-critical models displaying a short-lived double-dive behaviour. Head-on collisions of equal-mass stars yield three distinct outcomes -- boson star remnants, black hole formation at contact, and collapse of each star to a black hole prior to contact. The associated gravitational-wave energies reflect a competition between increasing compactness, which enhances the efficiency of gravitational-wave emission, and decreasing tidal deformability, which suppresses merger asymmetries, and at large self-interaction strengths the collapse-before-contact branch exhibits a pronounced non-monotonic structure. The simulations reported here constitute a substantial catalogue of initial conditions and waveforms, providing a natural basis for constructing surrogate models capable of rapidly predicting gravitational-wave signals across an extended parameter space.