Dynamical formation and stability of fermion-boson stars

TL;DR

This paper explores the formation and stability of fermion-boson stars, gravitationally bound structures of fermions and scalar particles, through numerical simulations of the Einstein-Klein-Gordon-Euler system under spherical symmetry.

Contribution

It introduces a scenario for fermion-boson star formation starting from a neutron star with an accreting scalar field, demonstrating the system's evolution to equilibrium and the role of fermionic cores in stability.

Findings

Fermion-boson stars can form via gravitational cooling from initial scalar field configurations.

Configurations with large scalar self-interaction constants develop nodes in the scalar field.

Fermionic cores may stabilize excited states with scalar field nodes.

Abstract

Gravitationally bound structures composed by fermions and scalar particles known as fermion-boson stars are regular and static configurations obtained by solving the coupled Einstein-Klein-Gordon-Euler (EKGE) system. In this work, we discuss one possible scenario through which these fermion-boson stars may form by solving numerically the EKGE system under the simplifying assumption of spherical symmetry. Our initial configurations assume an already existing neutron star surrounded by an accreting cloud of a massive and complex scalar field. The results of our simulations show that once part of the initial scalar field is expelled via gravitational cooling the system gradually oscillates around an equilibrium configuration that is asymptotically consistent with a static solution of the system. The formation of fermion-boson stars for large positive values of the coupling constant in the self-interaction term of the scalar-field potential reveal the presence of a node in the scalar field. This suggests that a fermionic core may help stabilize configurations with nodes in the bosonic sector, as happens for purely boson stars in which the ground state and the first excited state coexist.