Thermal Quarkyonic Matter and Its Implications for Neutron Star Structure

TL;DR

This paper tackles the open question of neutron-star matter composition by developing a finite-temperature quarkyonic matter model, extending previous cold formulations to include thermal effects. The authors implement a two-component EOS with non-interacting quarks occupying low-momentum states and neutrons occupying a momentum shell that shifts with temperature, enforcing chemical equilibrium and charge neutrality to produce the full thermodynamic description. They show that temperature can dramatically reshape the QM EOS, especially at low transition densities, leading to larger radii and tidal deformabilities and thus affecting mass-radius relations and gravitational-wave observables. The work provides a framework to constrain QM parameters using astrophysical data from pulsars and GW observations, with implications for proto-neutron stars, mergers, and cooling scenarios.

Abstract

The structure and basic properties of dense nuclear matter still remain one of the open problems of Physics. In particular, the composition of the matter that composes neutron stars is under theoretical and experimental investigation. Among the theories that have been proposed, apart from the classical one where the composition is dominated by hadrons, the existence or coexistence of deconfined quark matter is a dominant guess. An approach towards this solution is the phenomenological view according to which the existence of quarkyonic matter plays a dominant role in the construction of the equation of state (EOS). According to it the structure of the EOS is based on the existence of the quarkyonic particle which is a hybrid state of a particle that combines properties of hadronic and quark matter with a corresponding representation in momentum space. In this paper we propose a phenomenological model for hot quarkyonic matter, borrowed from corresponding applications in hadronic models, where the interaction in the quarkyonic matter depends not only on the position but also on the momentum of the quarkyonic particles. This consideration, as we demonstrate, can have a remarkable consequence on the shape of the EOS and thus on the properties of neutron stars, especially in those for which the effect of temperature is significant, offering a sufficiently flexible model. Comparison with recent observational data can place constraints on the parameterization of the particular model and help improve its reliability.

Figures (7)

• Figure 1: The momentum space represenation of quarkyonic matter at zero temperature (left) and at finite temperature (right) respectively. Quarks occupy all states from zero momentum up to maximum momentum ($h_d$ and $h_u$) for the down and up quark respectively). At higher momentum, quarks are confined into baryons that occupy momentum states in a shell with width $\Delta$ (see also the text and Ref. PhysRevLett.122.122701 for more details). As the temperature increases, quarks spread out to higher momentum states and the width of the shell where quarks are confined into nucleons decreases.
• Figure 2: The Fermi - Dirac distribution functions for quarks and neutrons, in quarkyonic matter, at total baryon density $n_B = 0.3$ fm$^{-3}$ and for several values of the temperature.
• Figure 3: The width of the momentum shell $\Delta$ as a function of the total baryon density $n_B$, for several values of the temperature, for $\kappa =0.3$, $N_c = 3$ and $\Lambda_{Qyc} = 200$ MeV .
• Figure 4: Neutron, up and down quark number densities as a fraction of the total baryon density, for cold quarkyonic matter and for finite temperature quarkyonic matter for $T = 5, 10 \ {\rm MeV}$.
• Figure 5: Equations of state for pure neutron matter and quarkyonic matter with: (a) $n_{\rm tr}=0.2\ {\rm fm}^{-3}$ and (b) $0.3 \ {\rm fm}^{-3}$, for several values of temperature.