Can we decipher the composition of the core of a neutron star?

Understand

General relativity guarantees a unique one-to-one correspondence between static observables of neutron stars (NSs) accessible by multi-messenger astronomy, such as mass-radius or tidal deformability, and the equation of state (EoS) of beta equilibrated matter.

• It is well known that these static properties are not enough to discern conventional NSs from hybrid stars.
• However, if one assumes that hadrons present in the neutron star core are only neutrons and protons, the lepton fraction is in principle determined unequivocally by the condition of chemical equilibrium.
• Using a simple analytical method based on a polynomial expansion of the EoS, we show that multiple solutions are possible to the beta-equilibrium equation, leading to a characteristic indetermination on the composition of the interiors of NSs, even in the purely nucleonic hypothesis.