Stability and collisions of excited spherical boson stars: glimpses of chains and rings

Abstract

Scalar, spherically symmetric, radially excited boson stars were previously shown to be stabilized, against spherical dynamics, by sufficiently strong self-interactions. Here, we further test their stability now in a full 3+1D evolution. We show that the stable stars in the former case become afflicted by a non-spherical instability. Then, we perform head-on collisions of both (stable) fundamental and (sufficiently long-lived) excited boson stars. Depending on the stars chosen, either a black hole or a bosonic remnant are possible. In particular, collisions of excited stars result in a bosonic bound state which resembles a dynamical superposition of chains and rings, akin to the ones found as equilibrium solutions in Liang:2025myf. These evolutions emphasize a key difference concerning the dynamical robustness of fundamental vs. excited spherical boson stars, when generic (beyond spherical) dynamics is considered.