Impact of quark deconfinement in neutron star mergers and hybrid star mergers

We describe an unambiguous gravitational-wave signature to identify the occurrence of a strong phase transition from hadronic matter to deconfined quark matter in neutron star mergers. Such a phase transition leads to a strong softening of the equation of state and hence to more compact merger remnants compared to purely hadronic models. If a phase transition takes place during merging, this results in a characteristic increase of the dominant postmerger gravitational-wave frequency relative to the tidal deformability characterizing the inspiral phase. By comparing results from different purely hadronic and hybrid models we show that a strong phase transition can be identified from a single, simultaneous measurement of pre- and postmerger gravitational waves. Furthermore, we present new results for hybrid star mergers, which contain quark matter already during the inspiral stage. Also for these systems we find that the postmerger GW frequency is increased compared to purely hadronic models. We thus conclude that also hybrid star mergers with an onset of the hadron-quark phase transition at relatively low densities may lead to the very same characteristic signature of quark deconfinement in the postmerger GW signal as systems undergoing the phase transition during merging.

Phases of Hadron-Quark Matter in (Proto) Neutron Stars

In the first part of this paper, we investigate the possible existence of a structured hadron-quark mixed phase in the cores of neutron stars. This phase, referred to as the hadron-quark pasta phase, consists of spherical blob, rod, and slab rare phase geometries. Particular emphasis is given to modeling the size of this phase in rotating neutron stars. We use the relativistic mean-field theory to model hadronic matter and the non-local three-flavor Nambu–Jona-Lasinio model to describe quark matter. Based on these models, the hadron-quark pasta phase exists only in very massive neutron stars, whose rotational frequencies are less than around 300 Hz. All other stars are not dense enough to trigger quark deconfinement in their cores. Part two of the paper deals with the quark-hadron composition of hot (proto) neutron star matter. To this end we use a local three-flavor Polyakov–Nambu–Jona-Lasinio model which includes the ’t Hooft (quark flavor mixing) term. It is found that this term leads to non-negligible changes in the particle composition of (proto) neutron stars made of hadron-quark matter.

Quark deconfinement in neutron stars and astrophysical implications

We investigate the quark deconfinement phase transition in cold ([Formula: see text]) and hot [Formula: see text]-stable hadronic matter. Assuming a first-order phase transition, we calculate and compare the nucleation rate and the nucleation time due to quantum and thermal nucleation mechanisms. We show that above a threshold value of the central pressure a pure hadronic star (HS) (i.e. a compact star with no fraction of deconfined quark matter (QM)) is metastable to the conversion to a quark star (QS) (i.e. a hybrid star or a strange star). This process liberates a huge amount of energy, of the order of 10[Formula: see text] erg, which produces a powerful neutrino burst, likely accompanied by intense gravitational waves emission, and possibly by a second delayed (with respect to the supernova explosion forming the HS) explosion which could be the energy source of a powerful gamma-ray burst (GRB). This stellar conversion process populates the QS branch of compact stars, thus one has in the universe two coexisting families of compact stars: HSs and QSs. We introduce the concept of critical mass [Formula: see text] for cold HSs and proto-hadronic stars (PHSs), and the concept of limiting conversion temperature for PHSs. We show that PHSs with a mass [Formula: see text] could survive the early stages of their evolution without decaying to QSs. Finally, we discuss the possible evolutionary paths of PHSs.