Quarkyonic matter with strangeness in an extended RMF model

TL;DR

The paper addresses the hadron-to-quark transition in dense neutron-star matter by formulating a unified quarkyonic framework that combines a relativistic mean-field description for baryons with an equivparticle model for quarks, including strangeness through $\Lambda$, $\Xi$, and $\Sigma$ hyperons and strange quarks. A quark Fermi sea coexists with a baryon Fermi surface, with densities and single-particle energies determined self-consistently under density-dependent masses $m_b(n_{ m b}^{\rm Q})$ and $m_q(n_{ m b})$, and with interface matching conditions linking the baryon and quark sectors. The study leverages three hadronic functionals (TW99, PKDD, DD-ME2) and parameter sets $(B, C, \sqrt{D})$ to explore how hyperon emergence and the quarkyonic transition soften the EOS, reducing the speed of sound to $v_{\max} \approx 0.6\,c$ and lowering $M_{\rm TOV}$ to about $2\,M_\odot$ in some cases, achieving consistency with current astrophysical constraints. Overall, the work provides a coherent framework that connects microphysical mechanisms to neutron-star observables and suggests that strangeness and quarkyonic dynamics can mitigate the hyperon puzzle while remaining compatible with observed massive pulsars.

Abstract

Quarkyonic matter is expected to play a key role for the transition from hadronic matter to quark matter in compact stars. Within the framework of the relativistic mean field (RMF) model and equivparticle model with density-dependent quark masses, we construct the ``quark Fermi sea" with a ``baryon Fermi surface" to characterize the properties of the quarkyonic matter. In particular, we develop a comprehensive framework to account for the strangeness degrees of freedom, incorporating $Λ$, $Ξ$, and $Σ$ hyperons as well as strange quarks in a unified quarkyonic framework. Our calculations indicate that the inevitable emergence of hyperons softens the equations of state, leading to a reduction in the speed of sound around $n_{\rm b}\approx 2n_0$, and consequently reducing the masses and radii of neutron stars. When the quark-hadron phase transition is taken into account, the equation of state at high densities exhibits additional softening, leading to a maximum sound velocity of $v_{\rm max} \approx 0.6\,c$, which is close to the ultrarelativistic limit of $0.58\,c$, consistent with current astronomical observational constraints.

Figures (5)

• Figure 1: Particle number densities $n_{i}$ for baryons ($n$, $p$, $\Lambda$, $\Xi$, $\Sigma$) and leptons ($e$, $\mu$) in neutron star matter as a function of the total baryon number density $n_{\rm b}=n_{\rm b}^{\rm B}$. Results are shown for three compositional models: nucleons and leptons only (solid lines), with the addition of $\Lambda$ hyperons (dashed lines), and with $\Lambda$, $\Xi$, and $\Sigma$ hyperons (dash-dotted lines). Panels (a), (b), and (c) correspond to calculations based on the TW99 NPA1999Typel_656_331, PKDD PRC2004Long_69_034319, and DD-ME2 PRC2005Lalazissis_71_024312 density functionals, respectively.
• Figure 2: Particle number densities $n_{i}$ for baryons ($n$, $p$, $\Lambda$, $\Xi$, $\Sigma$), quarks ($u$, $d$, $s$), and leptons ($e$, $\mu$) in quarkyonic matter as a function of the total baryon number density $n_{\rm b}=n_{\rm b}^{\rm B}+n_{\rm b}^{\rm Q}$. The parameter sets ($B$, $C$, $\sqrt{D}$) employed based on TW99 NPA1999Typel_656_331, PKDD PRC2004Long_69_034319, and DD-ME2 PRC2005Lalazissis_71_024312 density functionals are (300, 0.7, 180), (150, 0.7, 150), and (100, 0.5, 160), respectively.
• Figure 3: Energy per baryon $E/n_{\rm b}$ as a function of the total baryon number density $n_{\rm b}$ for nuclear matter (black lines) composted with nucleons and leptons only (solid lines), the further addition of $\Lambda$ hyperons (dashed lines), and the full inclusion of $\Lambda$, $\Xi$, and $\Sigma$ hyperons (dash-dotted lines), and quarkyonic matter (colored lines). These results are obtained using the parameter sets listed in Table \ref{['Table3']}.
• Figure 4: The same as Fig. \ref{['Fig3Enb']}, but for the velocity of sound $v$.
• Figure 5: Mass-radius relations of compact stars derived EOS presented in Fig. \ref{['Fig3Enb']}. The shaded regions represent the constraints from the binary neutron star merger event GW170817 ($90\%$ credible region) PRL2018Abbott_121_161101, as well as the observational pulse profiles of PSR J0030+0451 ($68\%$ credible region) AJL2019Riley_887_L21AJL2019Miller_887_L24, PSR J0740+6620 ($68\%$ credible region) AJL2021Riley_918_L27AJL2021Miller_918_L28, and PSR J0614-3329 (inner and outer regions corresponding to the 68% and 95% credible regions, respectively) ApJ2025Mauviard_995_60.