Irina Sagert

Signals of the QCD phase transition in core-collapse supernovae

We explore the implications of the QCD phase transition during the postbounce evolution of core-collapse supernovae. Using the MIT bag model for the description of quark matter, we model phase transitions that occur during the early postbounce evolution. This stage of the evolution can be simulated with general relativistic three-flavor Boltzmann neutrino transport. The phase transition produces a second shock wave that triggers a delayed supernova explosion. If such a phase transition happens in a future galactic supernova, its existence and properties should become observable as a second peak in the neutrino signal that is accompanied by significant changes in the energy of the emitted neutrinos. This second neutrino burst is dominated by the emission of antineutrinos because the electron degeneracy is reduced when the second shock passes through the previously neutronized matter.

Mass ejection by strange star mergers and observational implications

We determine the Galactic production rate of strangelets as a canonical input to calculations of the measurable cosmic ray flux of strangelets by performing simulations of strange star mergers and combining the results with recent estimates of stellar binary populations. We find that the flux depends sensitively on the bag constant of the MIT bag model of QCD and disappears for high values of the bag constant and thus more compact strange stars. In the latter case, strange stars could coexist with ordinary neutron stars as they are not converted by the capture of cosmic ray strangelets. An unambiguous detection of an ordinary neutron star would then not rule out the strange matter hypothesis.

Detecting the QCD phase transition in the next Galactic supernova neutrino burst

Predictions of the thermodynamic conditions for phase transitions at high baryon densities and large chemical potentials are currently uncertain and largely phenomenological. Neutrino observations of core-collapse supernovae can be used to constrain the situation. Recent simulations of stellar core collapse that include a description of quark matter predict a sharp burst of {nu}{sub e} several hundred milliseconds after the prompt {nu}{sub e} neutronization burst. We study the observational signatures of that {nu}{sub e} burst at current neutrino detectors--IceCube and Super-Kamiokande. For a Galactic core-collapse supernova, we find that signatures of the QCD phase transition can be detected, regardless of the neutrino oscillation scenario. The detection would constitute strong evidence of a phase transition in the stellar core, with implications for the equation of state at high matter density and the supernova explosion mechanism.

Compact stars for undergraduates

We report on an undergraduate student project initiated in the summer semester of 2004 with the aim to establish equations of state for white dwarfs and neutron stars for computing mass–radius relations as well as corresponding maximum masses. First, white dwarfs are described by a Fermi gas model of degenerate electrons and neutrons, and effects from general relativity are examined. For neutron star matter, the influence of a finite fraction of protons and electrons and of strong nucleon–nucleon interactions are studied. The nucleon–nucleon interactions are introduced within a Hartree–Fock scheme using a Skyrme-type interaction. Finally, masses and radii of neutron stars are computed for a given central pressure.

Strange quark matter in explosive astrophysical systems

Explosive astrophysical systems, such as supernovae or compact star binary mergers, provide conditions where strange quark matter can appear. The high degree of isospin asymmetry and temperatures of several MeV in such systems may cause a transition to the quark phase already around saturation density. Observable signals from the appearance of quark matter can be predicted and studied in astrophysical simulations. As input in such simulations, an equation of state with an integrated quark matter phase transition for a large temperature, density and proton fraction range is required. Additionally, restrictions from heavy ion data and pulsar observation must be considered. In this work we present such an approach. We implement a quark matter phase transition in a hadronic equation of state widely used for astrophysical simulations and discuss its compatibility with heavy ion collisions and pulsar data. Furthermore, we review the recently studied implications of the QCD phase transition during the early post-bounce evolution of core-collapse supernovae and introduce the effects from strong interactions to increase the maximum mass of hybrid stars. In the MIT bag model, together with the strange quark mass and the bag constant, the strong coupling constant αs provides a parameter to set the beginning and extension of the quark phase and with this the mass and radius of hybrid stars.

Is a soft nuclear equation of state extracted from heavy-ion data incompatible with pulsar data?

We discuss the recent constraints on the nuclear equation of state (EoS) from pulsar mass measurements and from subthreshold production of kaons in heavy-ion collisions. While the recent pulsar data point towards a hard EoS, the analysis of the heavy-ion data allows only for soft equations of state. We resolve the apparent contradiction by considering the different density regimes probed. We argue that future measurements of global properties of low-mass pulsars can serve as an excellent cross-check to the heavy-ion data.

Signals of the QCD phase transition in core collapse supernovae—microphysical input and implications on the supernova dynamics

In contrast to heavy ion collisions, matter in astrophysical systems such as neutron stars, compact star mergers and supernova environments can be highly isospin asymmetric. We focus on core collapse supernova matter where temperatures reach tens of MeV. Both conditions, the high temperatures and isospin asymmetry, can favour an early phase transition to quark matter already close to nuclear saturation density. We examine the QCD phase transition during the early postbounce phase of core collapse supernovae. We discuss the microphysical input, i.e. the modelling of the phase transition to strange quark matter, and the consequences on the dynamical evolution as well as the observable neutrino signal from the phase transition. The equation of state for strange quark matter is based on the MIT bag model. The phase transition is constructed applying the Gibbs criterion which results in an extended coexistence region in the phase diagram between the hadronic and the quark phases, i.e. the mixed phase. The supernovae are simulated via general relativistic radiation hydrodynamics based on three-flavour Boltzmann neutrino transport in spherical symmetry. The dynamical evolution of the phase transition to quark matter is determined by an adiabatic collapse due to the softening of the equation of state in the mixed phase. The equation of state for the pure quark phase stiffens again which causes the collapse to halt and a shock wave forms at the boundary between the mixed and the pure hadronic phases. This shock accelerates and launches an explosion, which releases a burst of neutrinos dominated by electron anti-neutrinos due to the lifted degeneracy of the shock-heated hadronic material.

Core collapse supernovae in the QCD phase diagram

We compare two classes of hybrid equations of state with a hadron-to-quark matter phase transition in their application to core collapse supernova simulations. The first one uses the quark bag model and describes the transition to three-flavor quark matter at low critical densities. The second one employs a Polyakov-loop extended Nambu-Jona-Lasinio (PNJL) model with parameters describing a phase transition to two-flavor quark matter at higher critical densities. These models possess a distinctly different temperature dependence of their transition densities which turns out to be crucial for the possible appearance of quark matter in supernova cores. During the early post-bounce accretion phase quark matter is found only if the phase transition takes place at sufficiently low densities as in the study based on the bag model. The increase critical density with increasing temperature, as obtained for our PNJL parametrization, prevents the formation of quark matter. The further evolution of the core collapse supernova as obtained applying the quark bag model leads to a structural reconfiguration of the central protoneutron star where, in addition to a massive pure quark matter core, a strong hydrodynamic shock wave forms and a second neutrino burst is released during the shock propagation across the neutrinospheres. We discuss the severe constraints in the freedom of choice of quark matter models and their parametrization due to the recently observed 2M⊙ pulsar and their implications for further studies of core collapse supernovae in the QCD phase diagram.

Strange matter in core-collapse supernovae

We discuss the possible impact of strange quark matter on the evolution of core-collapse supernovae with emphasis on low critical densities for the quark-hadron phase transition. For such cases the hot proto-neutron star can collapse to a more compact hybrid star configuration hundreds of milliseconds after core-bounce. The collapse triggers the formation of a second shock wave. The latter leads to a successful supernova explosion and leaves an imprint on the neutrino signal. These dynamical features are discussed with respect to their compatibility with recent neutron star mass measurements which indicate a stiff high density nuclear matter equation of state.

Is there quark matter in (low-mass) pulsars?

The effect of the QCD phase transition is studied for the mass–radius relation of compact stars and for hot and dense matter at a given proton fraction used as input in core-collapse supernova simulations. The phase transitions to the 2SC and CFL colour superconducting phases lead to stable hybrid star configurations with a pure quark matter core. In supernova explosions quark matter could be easily produced due to β equilibrium, small proton fractions and nonvanishing temperatures. A low critical density for the phase transition to quark matter is compatible with present pulsar mass measurements.