Head-on collisions of $\ell$-boson stars

Abstract

Fully non-linear numerical evolutions of the Einstein-(multi)--Klein-Gordon equations are performed to study head-on collisions of $\ell$-boson stars. Despite being spherically symmetric, $\ell$-boson stars have a (hidden) frame of reference, used in defining their individual multipolar fields. To assess the impact of their relative orientation, we perform simulations with different angles between the axes of the two colliding stars. Additionally, two scenarios are considered for the colliding stars: that they are composites of either the same or different scalar fields. Despite some model-specific behaviours, the simulations generically indicate that: 1) the collision of two sufficiently (and equally) massive stars leads to black hole formation; 2) below a certain mass threshold the end result of the evolution is a bound state of the composite scalar fields, that neither disperses nor collapses into a black hole within the simulation time; 3) this end product (generically) deviates from spherical symmetry and the equipartition of the number of bosonic particles between the different scalar fields composing the initial boson stars is lost, albeit not dramatically. This last observation indicates, albeit without being conclusive, that the end result of these collisions belongs to the previously reported larger family of equilibrium multi-field boson stars, generically non-spherical, and of which $\ell$-boson stars are a symmetry enhanced point. We also extract and discuss the waveforms from the collisions studied.

23 pages. Updated version with broader discussion, typos corrected and new analyses and figures

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• Self-interacting dipolar boson stars and their dynamics
• Boson star head-on collisions with constraint-violating and constraint-satisfying initial data
• On the radial linear stability of nonrelativistic $\ell$-boson stars
• Are nonrelativistic ground state $\ell$-boson stars only stable for $\ell =0$ and $\ell =1$?
• Eccentric mergers of binary Proca stars
• Massive boson stars: Stability and GW emission in head-on mergers
• Noble gravitational atoms: Self-gravitating black hole scalar wigs with angular momentum number
• $\ell$-Boson stars in anti-de Sitter spacetime
• Constraint-satisfying binary boson star initial data via XCFC
• Piercing of a boson star by a black hole