ETD

• astrophysics
• dark matter
• hyperon
• neutron star
• relativistic mean field theory

Neutron stars serve as exceptional astrophysical laboratories for exploring matter un￾der extreme densities. The potential presence of dark matter (DM) within these compact
objects offers valuable insights into how non-baryonic components may influence stellar
structure through purely gravitational coupling.
This thesis investigates the impact of an asymmetric fermionic dark-matter compo￾nent, modeled as a cold, non-self-interacting Fermi gas with particle masses in the GeV
range, on hyperonic neutron stars. The analysis is carried out within a two-fluid Tol￾man–Oppenheimer–Volkoff framework, where dark matter and baryonic matter interact
solely via gravity. The baryonic sector is described using the relativistic mean-field (RMF)
approach with the FSU2H parametrization, in both its nucleonic and hyperonic versions,
which consistently reproduce empirical nuclear properties and current astrophysical con￾straints from two-solar-mass pulsars, NICER radius estimates, and the tidal-deformability
limits inferred from GW170817.
In the purely baryonic case, the maximum mass reaches Mmax ≃ 2.38 M⊙ for nucleonic
stars and Mmax ≃ 2.05 M⊙ once hyperons are included. The addition of a dark-matter
component introduces an enhanced gravitational pull on the ordinary-matter fluid, lead￾ing to a systematic reduction in the maximum gravitational mass of the resulting DM￾admixed hyperonic star sequence compared to its purely hyperonic counterpart. This
mass reduction scales with the DM particle mass and exhibits an almost linear depen￾dence for small dark-matter fractions fχ. Maintaining the observational 2 M⊙ constraint
requires fχ ≲ 3%.
Specific static configurations were examined at fixed stellar mass and dark-matter
fraction to illustrate the internal rearrangement of matter. The dark component tends to
form a compact core whose radius decreases with increasing DM particle mass, while the
baryonic density redistributes accordingly. As a consequence, the onset of hyperons shifts
to larger radii, closer to the stellar surface, thereby extending the hyperonic core region.
The configurations calculated for the considered range of DM particle masses and
small dark-matter fractions remain consistent with current multimessenger constraints,
suggesting that such systems are astrophysically viable.
Overall, this study establishes a coherent framework connecting dense-matter physics,
dark-matter phenomenology, and relativistic astrophysics, and shows that even a small
dark-matter admixture can modify the global structure and hyperonic composition of
neutron stars, with potential implications for their evolution and thermal history.