Vector boson stars, also known as Proca stars, exhibit remarkable dynamical robustness, making them strong candidates for potential astrophysical exotic compact objects. In search of theoretically well-motivated Proca star models, we recently introduced the $\ell$-Proca star, a multifield extension of the spherical Proca star, whose $(2\ell +1)$ constitutive fields have the same time and radial dependence, and their angular structure is given by all the available spherical harmonics for a fixed angular momentum number $\ell$. In this work, we conduct a nonlinear stability analysis of these stars by numerically solving the Einstein- (multi-, complex) Proca system for the case of $\ell =2$, which are formed by five constitutive independent, complex Proca fields with $m=0,|1|$, and $|2|$. Our analysis is based on long-term, fully nonlinear, three-dimensional numerical-relativity simulations without imposing any symmetry. We find that ($\ell =2$)-Proca stars are unstable throughout their entire domain of existence. In particular, we highlight that less compact configurations dynamically lose their global spherical symmetry, developing a nonaxisymmetric $\tilde{m}=4$ mode instability and a subsequent migration into a new kind of multifield Proca star formed by fields with different angular momentum number, $\ell =1$ and $\ell =2$, that we identify as unstable multi-$\ell$-Proca stars.

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• Fluids & classical fields in curved spacetime
• Gravitation
• Numerical relativity
• Numerical simulations in gravitation & astrophysics