Signatures of $K^-$ condensation on neutron star structure and $f-$mode frequencies

Abstract

Antikaon ($K^-$) condensation within neutron star matter (NS) depends on the antikaon-nucleon interaction potential ($U_K$). Appearance of $K^-$ generally softens the equation of state (EOS). The impact of this softening on the structure of the NS can be leveraged to find a telltale sign of the phase transition from nucleonic matter to $K^-$ condensation. To investigate the impact of $K^-$ condensation on NS properties using a Bayesian inference framework, we choose two sets of RMF model parameters to obtain a stiff (DD2) and relatively soft (FSU) nucleonic EOS, and explore a wide range of optical potential depths. Multimessenger observations from NICER and LIGO/Virgo constrain the optical potential values to $U_K = -104.72^{+13.82}_{-12.48}$ MeV and $U_K = -66.46^{+2.47}_{-3.42}$ MeV for the stiff and soft cases, respectively. Deeper $K^-$ potentials trigger condensation at a lower density, softening the EOS and lowering the corresponding maximum masses. While slopes of mass-radius and tidal deformability curves overlap between nucleonic and exotic EOSs, their curvature and $f-$mode oscillation properties (frequency and damping time) reveal features attributable to EOS softening. However, distinguishing the specific exotic degrees of freedom responsible for the softening remains an open challenge.