Treating quarks within neutron stars

Abstract

Neutron star interiors provide the opportunity to probe properties of cold dense matter in the QCD phase diagram. Utilizing models of dense matter in accord with nuclear systematics at nuclear densities, we investigate the compatibility of deconfined quark cores with current observational constraints on the maximum mass and tidal deformability of neutron stars. We explore various methods of implementing the hadron-to-quark phase transition, specifically, first-order transitions with sharp (Maxwell construction) and soft (Gibbs construction) interfaces, and smooth crossover transitions. We find that within the models we apply, hadronic matter has to be stiff for a first-order phase transition and soft for a crossover transition. In both scenarios and for the equations of state we employed, quarks appear at the center of pre-merger neutron stars in the mass range ≈ 1.0 − 1.6 M , with a squared speed of sound c 2 QM 0.4 characteristic of strong repulsive interactions required to support the recently discovered neutron star masses ≥ 2 M . We also identify equations of state and phase transition scenarios that are consistent with the bounds placed on tidal deformations of neutron stars in the recent binary merger event GW170817. We emphasize that distinguishing hybrid stars with quark cores from normal hadronic stars is very difficult from the knowledge of masses and radii alone, unless drastic sharp transitions induce distinctive disconnected hybrid branches in the mass-radius relation.

Impact of the PSR J0740+6620 radius constraint on the properties of high-density matter

“…On the other hand, a positive ΔR suggests weaker phase transitions, progressing toward the absolute upper bound on NS radii. We also note that for various physical models of hadronic matter (with or without a smooth crossover to exotic matter), ΔR ≳ −1.5 km is typical, although a few exhibit an increase from R 1.4 to R 2.0 [108,118,119].…”

Neutron-star tidal deformability and equation-of-state constraints

“…7), astronomical observations can also be used to place constraints on the relevant parameter space. In general, since GW170817 provided an upper limit onΛ, any physical effect that results in a softening of the equation of state can be consistent with the data [88,[240][241][242][243][244][245][246] As such, a strong first-order phase transition might make an equation of state model compatible with the GW170817 data, even if the hadronic part on its own is not [152,[247][248][249][250]. A number of studies have constructed models that can successfully interpret the inspiral signal from GW170817 as the coalescence of any combination of hadronic and hybrid hadronic-quark neutron stars and place corresponding constraints on the relevant model parameter space [149,247,248,[251][252][253][254][255][256][257][258][259][260].…”

Cold quark matter with heavy quarks and the stability of charm stars

“…Although charm stars are excluded by our analysis, and also due to causality limits posed by maximum mass constraints from neutron star observations [3,4], it is possible to have small amounts of charm quark matter in the core of the heaviest observed neutron stars (or, rather, hybrid stars), where a matching between a nuclear and a quark phase could be possible via the Glendenning construction for first-order phase transitions [46]. Recently, a related possibility was investigated under the consideration of strange quark matter contaminated by charm quark impurities (in the sense of condensed matter physics), producing a QCD Kondo effect [47,48].…”