We investigate nuclear matter at finite temperature and density, including the formation of light clusters up to the alpha particle The novel feature of this work is to include the formation of clusters as well as their dissolution due to medium effects in a systematic way using two many-body theories: a microscopic quantum statistical (QS) approach and a generalized relativistic mean field (RMF) model. Nucleons and clusters are modified by medium effects. Both approaches reproduce the limiting cases of nuclear statistical equilibrium (NSE) at low densities and cluster-free nuclear matter at high densities. The treatment of the cluster dissociation is based on the Mott effect due to Pauli blocking, implemented in slightly different ways in the QS and the generalized RMF approaches. We compare the numerical results of these models for cluster abundances and thermodynamics in the region of medium excitation energies with temperatures T <= 20 MeV and baryon number densities from zero to a few times saturation density. The effect of cluster formation on the liquid-gas phase transition and on the density dependence of the symmetry energy is studied. Comparison is made with other theoretical approaches, in particular those, which are commonly used in astrophysical calculations. The results are relevant for heavy-ion collisions and astrophysical applications.Comment: 32 pages, 15 figures, minor corrections, accepted for publication in Physical Review

A new scheme for testing nuclear matter equations of state (EoSs) at high densities using constraints from neutron star (NS) phenomenology and a flow data analysis of heavy-ion collisions is suggested. An acceptable EoS shall not allow the direct Urca process to occur in NSs with masses below 1.5M , and also shall not contradict flow and kaon production data of heavy-ion collisions. Compact star constraints include the mass measurements of 2.1 ± 0.2M (1σ level) for PSR J0751+1807 and of 2.0 ± 0.1M from the innermost stable circular orbit for 4U 1636-536, the baryon mass-gravitational mass relationships from Pulsar B in J0737-3039 and the mass-radius relationships from quasiperiodic brightness oscillations in 4U 0614+09 and from the thermal emission of RX J1856-3754. This scheme is applied to a set of relativistic EoSs which are constrained otherwise from nuclear matter saturation properties. We demonstrate on the given examples that the test scheme due to the quality of the newly emerging astrophysical data leads to useful selection criteria for the high-density behavior of nuclear EoSs.

We identify an observable imprint of a first-order hadron-quark phase transition at supranuclear densities on the gravitational-wave (GW) emission of neutron star mergers. Specifically, we show that the dominant postmerger GW frequency f peak may exhibit a significant deviation from an empirical relation between f peak and the tidal deformability if a strong first-order phase transition leads to the formation of a gravitationally stable extended quark matter core in the postmerger remnant. A comparison of the GW signatures from a large, representative sample of microphysical, purely hadronic equations of state indicates that this imprint is only observed in those systems which undergo a strong first-order phase transition. Such a shift of the dominant postmerger GW frequency can be revealed by future GW observations, which would provide evidence for the existence of a strong first-order phase transition in the interior of neutron stars.PACS numbers: 04.30. Tv,26.60.Kp,26.60Dd,97.60.Jd Introduction: The theory of strong interactions, quantum chromodynamics (QCD), with quarks and gluons as fundamental degrees of freedom predicts a transition from nuclear matter to quark matter. At vanishing baryonic chemical potential, numerical solutions of QCD are available, which state a smooth crossover transition at a temperature of T = 154 ± 9 MeV [1][2][3]. At finite baryon densities only phenomenological models of QCD exist, which are benchmarked by nuclear matter phenomenology around nuclear saturation density ρ sat ≈ 2.7×10 14 g cm −3 [4] and by perturbative QCD at asymptotic densities [5]. Those methods, however, are not applicable in the region of the hadron-quark transition. Hence, the nature of the transition to quark matter (crossover or first-order phase transition) remains unclear. Whether the hadron-quark phase transition occurs at conditions which are found in compact stellar objects, e.g., in neutron stars (NS) with central densities of several times ρ sat , is presently unknown. The very first detection of gravitational waves (GW) from a NS merger [6] highlights the prospect to learn about the presence and the nature of the QCD phase transition in stellar objects, e.g. [7][8][9][10][11][12][13][14][15].

Abstract. We study the cooling of isolated neutron stars. The main cooling regulators are introduced: equation of state (EoS), thermal transport, heat capacity, neutrino and photon emissivity, superfluid nucleon gaps. The neutrino emissivity includes the main processes. A strong impact of medium effects on the cooling rates is demonstrated. Taking into account medium effects on reaction rates and on nucleon superfluid gaps modern experimental data can be well explained.

Aims. We present a new microscopic hadron-quark hybrid equation of state model for astrophysical applications, from which compact hybrid star configurations are constructed. These are composed of a quark core and a hadronic shell with a first-order phase transition at their interface. The resulting mass-radius relations are in accordance with the latest astrophysical constraints. Methods. The quark matter description is based on a quantum chromodynamics (QCD) motivated chiral approach with higher-order quark interactions in the Dirac scalar and vector coupling channels. For hadronic matter we select a relativistic mean-field equation of state with density-dependent couplings. Since the nucleons are treated in the quasi-particle framework, an excluded volume correction has been included for the nuclear equation of state at suprasaturation density which takes into account the finite size of the nucleons. Results. These novel aspects, excluded volume in the hadronic phase and the higher-order repulsive interactions in the quark phase, lead to a strong first-order phase transition with large latent heat, i.e. the energy-density jump at the phase transition, which fulfils a criterion for a disconnected third-family branch of compact stars in the mass-radius relationship. These twin stars appear at high masses (∼2 M ) that are relevant for current observations of high-mass pulsars.Conclusions. This analysis offers a unique possibility by radius observations of compact stars to probe the QCD phase diagram at zero temperature and large chemical potential and even to support the existence of a critical point in the QCD phase diagram.

A quantum kinetic equation is derived for the description of pair production in a timedependent homogeneous electric field E(t). As a source term, the Schwinger mechanism for particle creation is incorporated. Possible particle production due to collisions and collisional damping are neglected. The main result is a kinetic equation of non-Markovian character. In the low density approximation, the source term is reduced to the leading part of the well known Schwinger formula for the probability of pair creation. We discuss the momentum and time dependence of the derived source term and compare with other approaches. *

Gravitational wave observations of GW170817 placed bounds on the tidal deformabilities of compact stars allowing one to probe equations of state for matter at supranuclear densities. Here we design new parametrizations for hybrid hadron-quark equations of state, that give rise to low-mass twin stars, and test them against GW170817. We find that GW170817 is consistent with the coalescence of a binary hybrid star-neutron star. We also test and find that the I-Love-Q relations for hybrid stars in the third family agree with those for purely hadronic and quark stars within ∼ 3% for both slowly and rapidly rotating configurations, implying that these relations can be used to perform equation-of-state independent tests of general relativity and to break degeneracies in gravitational waveforms for hybrid stars in the third family as well.

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