# A stabilization mechanism for excited fermion-boson stars

**Authors:** Fabrizio Di Giovanni, Saeed Fakhry, Nicolas Sanchis-Gual, Juan Carlos Degollado, José A. Font  
**arXiv:** [2105.00530](https://arxiv.org/abs/2105.00530)  
**Date:** 2021-05-02  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2105.00530v1/

## Key points

- Investigating the nonlinear stability of excited fermion-boson stars in spherical symmetry
- Excited fermion-boson stars have at least one node in the radial profile of the scalar field
- Dynamical emergence of an excited fermion-boson star from the accretion of a cloud of scalar field onto a neutron star
- Constructing equilibrium configurations and analyzing stability properties using numerical-relativity simulations
- Cooperative stabilization mechanism between fermionic and bosonic constituents within mixed-state stars
- Purely gravitational effect, not dependent on the type of matter present in the companion star
- Comparison with stabilization mechanisms observed in other contexts such as $\ell-$boson stars and multi-field, multi-frequency boson stars
- Distinct stabilization mechanism specific to excited fermion-boson stars
- Valuable insights into stability properties of compound hypothetical stars composed of both fermions and bosons
- Shedding light on new aspects of stellar dynamics and potential avenues for further exploration in gravitational physics.

## Summary

In this study, the authors investigate the nonlinear stability of excited fermion-boson stars in spherical symmetry. These hypothetical stars are composed of both fermions and bosons and are described within the coupled Einstein-Klein-Gordon-Euler theoretical framework. The excited configurations of these stars are characterized by the presence of at least one node in the radial profile of the scalar field, which represents the bosonic component. The researchers begin by numerically revealing the dynamical emergence of an excited fermion-boson star from the accretion of a cloud of scalar field onto an already-formed neutron star, as observed in their previous investigation. Building upon this finding, they construct equilibrium configurations of excited fermion-boson stars and analyze their stability properties using numerical-relativity simulations. Contrary to purely boson stars in an excited state, which are known to be generally unstable, this study uncovers a cooperative stabilization mechanism between the fermionic and bosonic constituents within these mixed-state stars. This mechanism is found to be a purely gravitational effect and does not depend on the type of matter present in the companion star. The authors also compare their results with recent discussions on stabilization mechanisms observed in other contexts, such as $\ell-$boson stars and multi-field, multi-frequency boson stars. However, it is noted that their findings suggest a distinct stabilization mechanism specific to excited fermion-boson stars. Overall, this research provides valuable insights into the stability properties of compound hypothetical stars composed of both fermions and bosons. The discovery of a cooperative stabilization mechanism sheds light on new aspects of stellar dynamics and highlights potential avenues for further exploration in gravitational physics.