Effects of a Brueckner-Hartree-Fock-corrected effective mass on speed of sound, conformality, and observables of dark matter-admixed neutron stars

TL;DR

This work integrates a DM-admixed neutron-star EoS within an RMF framework, augmented by nucleon effective masses informed by relativistic Brueckner-Hartree-Fock calculations, to study macroscopic observables, the speed of sound, and conformality. The authors present a DM-augmented SU(2) chiral sigma model, derive the EoS with both RMF and BHF-informed mass treatments, and explore parameter choices including DM masses and BHF fitting functions. They show that BHF corrections lead to more compact stars, smaller radii at fixed mass, and a non-monotonic C_S^2 that trends toward the conformal limit at high density, all while remaining compatible with some observational bounds but facing tension with very massive pulsars. The results suggest that incorporating realistic many-body effects via BHF improves agreement with several observations for DM admixed NSs, but highlight the need for further development of DM interactions, isospin effects, and possible new degrees of freedom to reconcile heavy NS data with conformality constraints.

Abstract

We construct an equation of state describing cold and dense matter in the core of neutron stars which includes an admixture of fermionic dark matter and incorporates nucleon effective masses derived from the relativistic Brueckner-Hartree-Fock (BHF) many-body approach within a relativistic mean-field model. Such a BHF-informed mixed-model approach increases stellar compactness, with mass-radius configurations which are consistent with smaller, lighter pulsars. The model displays the expected non-monotonic behavior of sound speed hinted at by neutron star data, and is closer to the conformal bound at maximum mass. We find that the model displays tension with bounds on heavier pulsars, suggesting that the hypothesis of an aggregated dark component in neutron stars needs further critical study.

Figures (6)

• Figure 1: (a) Comparison of RMF and BHF corrected nucleon effective mass ratios for purely hadronic EoS. (b) The density profiles $\rho(r)/\rho_0$ for RMF and BHF hadronic $2.0 M_\odot$ NS. The higher density in the BHF case leads to a more compact NS.
• Figure 2: Comparison of EoSs for RMF and BHF corrected cases for BP II parameter set. The constraint $\Lambda_{1.4} < 400$ is taken from Ref. Annala:2017llu.
• Figure 3: (a) Mass-radius relationships for hadronic, BPI and BPII parameters in the RMF scenario. Theoretical results are compared with relevant experimental results presented for pulsars in Table \ref{['tab:Data']}. (b) Mass-radius relationships for hadronic, BPI and BPII parameters in the BHF scenario. The M-R curves in the BHF scenario are completely within the $\Lambda(1.4M_\odot) < 400$ constraint obtained from Annala:2017llu, which is consistent with the result in Fig.\ref{['fig:M-Lambda']}.
• Figure 4: Mass-radius relationships for hadronic, BPI and BPII parameters in the both RMF and BHF scenarios are compared with various GW waves data.
• Figure 5: Compactness ($C$), Tidal deformability ($\Lambda$) and MoI ($I$) of NSs as functions of mass ($M$ in $M_\odot$ units) are presented in panels (a), (b), and (c), respectively. Panel (d) shows the variation of $I$ with stellar radius ($R$). Results obtained for purely hadronic and DMANS (for BPI and BPII) cases in RMF and BHF scenarios are compared with the experimental findings presented in Table \ref{['tab:Data']}.