Dynamical formation and stability of fermion-boson stars

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 thesefermion-boson stars may form by solving numerically the EKGE system under the simplifying as-sumption of spherical symmetry. Our initial configurations assume an already existing neutron starsurrounded by an accreting cloud of a massive and complex scalar field. The results of our simula-tions show that once part of the initial scalar field is expelled via gravitational cooling the systemgradually oscillates around an equilibrium configuration that is asymptotically consistent with astatic solution of the system. The formation of fermion-boson stars for large positive values of thecoupling constant in the self-interaction term of the scalar-field potential reveal the presence of anode in the scalar field. This suggests that a fermionic core may help stabilize configurations withnodes in the bosonic sector, as happens for purely boson stars in which the ground state and thefirst excited state coexist.