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We present the first simulations of core-collapse supernovae (CCSNe) in axial symmetry (2D) with feedback from fast neutrino flavor conversion (FFC). Our schematic treatment of FFCs assumes instantaneous flavor equilibration under the constraint of lepton-number conservation. Systematically varying the spatial domain where FFCs are assumed to occur, we find that they facilitate SN explosions in low-mass (9-12 solar masses) progenitors that otherwise explode with longer time delays, whereas FFCs weaken the tendency to explode of higher-mass (around 20 solar masses) progenitors., Comment: 7 pages, 4 figures, submitted to PRL

We investigate the nucleosynthesis and kilonova properties of binary neutron-star (NS) merger models which lead to intermediate remnant lifetimes of ~0.1-1seconds until black-hole (BH) formation and describe all components of material ejected during the dynamical merger phase, NS-remnant evolution, and final viscous disintegration of the BH torus after gravitational collapse. To this end we employ a combination of hydrodynamics, nucleosynthesis, and radiative-transfer tools to achieve a consistent end-to-end modeling of the system and its observables. We adopt a novel version of the Shakura-Sunyaev scheme allowing to vary the approximate turbulent viscosity inside the NS remnant independently of the surrounding disk. We find that asymmetric progenitors lead to shorter remnant lifetimes and enhanced ejecta masses, although the viscosity affects the absolute values of these characteristics. The integrated production of lanthanides and heavier elements in such binary systems is sub-solar, suggesting that the considered scenarios contribute in a sub-dominant fashion to r-process enrichment. One reason is that BH-tori formed after delayed collapse exhibit less neutron-rich conditions than typically found, and often assumed in previous BH-torus models, for early BH formation. The outflows in our models feature strong anisotropy as a result of the lanthanide-poor polar neutrino-driven wind pushing aside lanthanide-rich dynamical ejecta. Considering the complexity of the models, the estimated kilonova light curves show promising agreement with AT2017gfo after times of several days, while the remaining inconsistencies at early times could possibly be overcome in binary configurations with a more dominant neutrino-driven wind relative to the dynamical ejecta., Comment: 16 pages, 9 figures, 1 table, accepted to ApJL

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The release of spin-down energy by a magnetar is a promising scenario to power several classes of extreme explosive transients. However, it lacks a firm basis because magnetar formation still represents a theoretical challenge. Using the first three-dimensional simulations of a convective dynamo based on a protoneutron star interior model, we demonstrate that the required dipolar magnetic field can be consistently generated for sufficiently fast rotation rates. The dynamo instability saturates in the magnetostrophic regime with the magnetic energy exceeding the kinetic energy by a factor of up to 10. Our results are compatible with the observational constraints on galactic magnetar field strength and provide strong theoretical support for millisecond protomagnetar models of gamma-ray burst and superluminous supernova central engines., 6 pages, 4 figures

The possible detection of a compact object in the remnant of SN 1987A presents an unprecedented opportunity to follow its early evolution. The suspected detection stems from an excess of infrared emission from a dust blob near the compact object's predicted position. The infrared excess could be due to the decay of isotopes like 44Ti, accretion luminosity from a neutron star or black hole, magnetospheric emission or a wind originating from the spindown of a pulsar, or thermal emission from an embedded, cooling neutron star (NS 1987A). It is shown that the last possibility is the most plausible as the other explanations are disfavored by other observations and/or require fine-tuning of parameters. Not only are there indications the dust blob overlaps the predicted location of a kicked compact remnant, but its excess luminosity also matches the expected thermal power of a 30 year old neutron star. Furthermore, models of cooling neutron stars within the Minimal Cooling paradigm readily fit both NS 1987A and Cas A, the next-youngest known neutron star. If correct, a long heat transport timescale in the crust and a large effective stellar temperature are favored, implying relatively limited crustal n-1S0 superfluidity and an envelope with a thick layer of light elements, respectively. If the locations don't overlap, then pulsar spindown or accretion might be more likely, but the pulsar's period and magnetic field or the accretion rate must be rather finely tuned. In this case, NS 1987A may have enhanced cooling and/or a heavy-element envelope., 21 pages, 6 figures, to be published in ApJ

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The neutrino process that occurs in the outer stellar shells during a supernova explosion and involves neutrino-nucleus reactions produces a range of rare, stable and radioactive isotopes. We improve previous $��$-process studies by using, for the first time, the time-dependent neutrino emission spectra, as predicted from supernova simulations, rather than a simplified parametric description modeled after the neutron-star cooling phase. In particular, our calculations use time-dependent neutrino spectra for all neutrino species, consider their deviation from a Fermi-Dirac distribution and account for the neutrino emission from the neutrino burst and accretion phases. We find that the time-dependent treatment of the neutrino emission spectra results in higher yields for the selected nuclei produced by the $��$~process as compared to previous studies and also compared to the approximation of assuming constant neutrino energies corresponding to the time-averaged mean energy radiated in each species. The effect is largest for nuclides produced by charged-current reactions. Our results reflect the dynamical competition between neutrino-induced reactions and the effect of the shock passage through the star. By varying the neutrino burst luminosity and the duration of the accretion phase, we study the impact of these early emission phases and their uncertainties on the $��$-process nucleosynthesis. We find that the deviation of the neutrino spectra from a Fermi-Dirac distribution calculated in supernova simulations has a negligible effect on the $��$-process yields., 16 pages, 8 figures, 2 tables, submitted to the Astrophysical Journal

We present a comparison between several simulation codes designed to study the core-collapse supernova mechanism. We pay close attention to controlling the initial conditions and input physics in order to ensure a meaningful and informative comparison. Our goal is three-fold. First, we aim to demonstrate the current level of agreement between various groups studying the core-collapse supernova central engine. Second, we desire to form a strong basis for future simulation codes and methods to compare to. Lastly, we want this work to be a stepping stone for future work exploring more complex simulations of core-collapse supernovae, i.e., simulations in multiple dimensions and simulations with modern neutrino and nuclear physics. We compare the early (first ~500ms after core bounce) spherically-symmetric evolution of a 20 solar mass progenitor star from six different core-collapse supernovae codes: 3DnSNe-IDSA, AGILE-BOLTZTRAN, FLASH, F{\sc{ornax}}, GR1D, and PROMETHEUS-VERTEX. Given the diversity of neutrino transport and hydrodynamic methods employed, we find excellent agreement in many critical quantities, including the shock radius evolution and the amount of neutrino heating. Our results provide an excellent starting point from which to extend this comparison to higher dimensions and compare the development of hydrodynamic instabilities that are crucial to the supernova explosion mechanism, such as turbulence and convection., Comment: 24 pages, 7 figures, J. Phys. G focus issue on core-collapse supernovae. This document was written collaboratively on Authorea, comments welcome at https://www.authorea.com/users/1943/articles/167397-global-comparison-of-core-collapse-supernova-simulations-in-spherical-symmetry

Neutrinos play a crucial role in the collapse and explosion of massive stars, governing the infall dynamics of the stellar core, triggering and fueling the explosion and driving the cooling and deleptonization of the newly formed neutron star. Due to their role neutrinos carry information from the heart of the explosion and, due to their weakly interacting nature, offer the only direct probe of the dynamics and thermodynamics at the center of a supernova. In this paper, we review the present status of modelling the neutrino physics and signal formation in collapsing and exploding stars. We assess the capability of current and planned large underground neutrino detectors to yield faithful information of the time and flavor dependent neutrino signal from a future Galactic supernova. We show how the observable neutrino burst would provide a benchmark for fundamental supernova physics with unprecedented richness of detail. Exploiting the treasure of the measured neutrino events requires a careful discrimination of source-generated properties from signal features that originate on the way to the detector. As for the latter, we discuss self-induced flavor conversions associated with neutrino-neutrino interactions that occur in the deepest stellar regions; matter effects that modify the pattern of flavor conversions in the dynamical stellar envelope; neutrino-oscillation signatures that result from structural features associated with the shock-wave propagation as well as turbulent mass motions in post-shock layers. Finally, we highlight our current understanding of the formation of the diffuse supernova neutrino background and we analyse the perspectives for a detection of this relic signal that integrates the contributions from all past core-collapse supernovae in the Universe., Comment: (revised version:116 pages, 46 pdf figures. References updated and typos removed. Matches the version published on La Rivista del Nuovo Cimento)

We have performed r-process calculations for matter ejected dynamically in neutron star mergers based on a complete set of trajectories from a three-dimensional relativistic smoothed particle hydrodynamic simulation. Our calculations consider an extended nuclear network, including spontaneous, $\beta$- and neutron-induced fission and adopting fission yield distributions from the ABLA code. We have studied the sensitivity of the r-process abundances to nuclear masses by using different models. Most of the trajectories, corresponding to 90% of the ejected mass, follow a relatively slow expansion allowing for all neutrons to be captured. The resulting abundances are very similar to each other and reproduce the general features of the observed r-process abundance (the second and third peaks, the rare-earth peak and the lead peak) for all mass models as they are mainly determined by the fission yields. We find distinct differences in the abundance yields at and just above the third peak, which can be traced back to different predictions of neutron separation energies for r-process nuclei around neutron number $N=130$. The remaining trajectories, which contribute 10% by mass to the total integrated abundances, follow such a fast expansion that the r process does not use all the neutrons. This also leads to a larger variation of abundances among trajectories as fission does not dominate the r-process dynamics. The total integrated abundances are dominated by contributions from the slow abundances and hence reproduce the general features of the observed r-process abundances. We find that at timescales of weeks relevant for kilonova light curve calculations, the abundance of actinides is larger than the one of lanthanides. Hence actinides can be even more important than lanthanides to determine the photon opacities under kilonova conditions. (Abridged), Comment: 17 pages, 7 figures, resubmitted to PRC addressing referee comments

The VERTEX code is employed for multi-dimensional neutrino-radiation hydrodynamics simulations of core-collapse supernova explosions from first principles. The code is considered state-of-the-art in supernova research and it has been used for modeling for more than a decade, resulting in numerous scientific publications. The computational performance of the code, which is currently deployed on several high-performance computing (HPC) systems up to the Tier-0 class (e.g. in the framework of the European PRACE initiative and the German GAUSS program), however, has so far not been extensively documented. This paper presents a high-level overview of the relevant algorithms and parallelization strategies and outlines the technical challenges and achievements encountered along the evolution of the code from the gigaflops scale with the first, serial simulations in 2000, up to almost petaflops capabilities, as demonstrated lately on the SuperMUC system of the Leibniz Supercomputing Centre (LRZ). In particular, we shall document the parallel scalability and computational efficiency of VERTEX at the large scale and on the major, contemporary HPC platforms. We will outline upcoming scientific requirements and discuss the resulting challenges for the future development and operation of the code., Comment: 10 pages, 2 figures, conference procceding

Müller, Bernhard "Core-collapse supernovae: Explosion dynamics, neutrinos and gravitational waves" in Hamburg neutrinos from supernova explosions. Proceedings, Workshop, HANSE 2011 / Mirizzi, Alessandro (eds.), Verlag Deutsches Elektronen-Synchrotron : 2011 ; HAνSE 2011 : Hamburg Neutrinos From Supernova Explosions, 2011-07-19 - 2011-07-23, Hamburg Hamburg Neutrinos From Supernova Explosions, HAνSE 2011, Hamburg, Germany, 19 Jul 2011 - 23 Jul 2011 ; DESY-PROC 14-21(2011). doi:10.3204/DESY-PROC-2011-03/mueller, The quest for the supernova explosion mechanism has been one of the outstanding challenges in computational astrophysics for several decades. Simulations have now progressed to a stage at which the solution appears close and neutrino and gravitational wave signals from self-consistent explosion models are becoming available. Here we focus one of the recent advances in supernova modeling, the inclusion of general relativity in multi-dimensional neutrino hydrodynamics simulations, and present the latest simulation results for an 11.2M(sun) and a 15M(sun) progenitor. We also mention 3D effects as another aspect in supernova physics awaiting further, more thorough investigation., Published by Verlag Deutsches Elektronen-Synchrotron, Hamburg

Context. The influence of magnetic fields on stellar core collapse and explosion is not well explored. It depends on the possibility to amplify the pre-collapse fields. Without rotation this can happen by compression, convection, the standing accretion shock instability, and the accumulation and growth of Alfven waves in the accretion flows. Aims. We investigate such amplification mechanisms of the magnetic field during the collapse and post-bounce evolution of the core of a non-rotating 15 solar mass star with varied initial field strengths, taking into account the microphysical equation of state and neutrino physics crucial for supernova cores. Methods. We perform simulations of ideal MHD with neutrino transport in axisymmetry. The transport of electron neutrinos and antineutrinos is treated with a new scheme that solves the energy-dependent set of radiation energy and momentum equations in 2d by using an analytic closure relation. Results. The magnetic field undergoes amplification by turbulent flows. We find indications for amplification by interacting waves in accretion streams. The fields can reach up to equipartition with the velocity field. Very high magnetic field strengths require very strong pre-collapse fields and are able to shape the post-bounce flow, leading to a pattern dominated by low-order multipoles. Such models are closest to a successful explosion. Conclusions. Magnetic fields can build up to interesting strengths even in non-rotating collapsing stellar cores. Starting with fields in the pre-collapse core as predicted by present stellar evolution models, typical neutron star fields emerge, whereas stronger progenitor fields lead to fields of magnetar strength. Only in the latter case the fields have dynamical effects on the flows in the supernova core. However, in none of our 2D simulations, we find an explosion until 500 ms post-bounce., Submitted to Astronomy & Astrophysics. 19 pages, 17 figures; quality of figures reduced

We present results from the first generation of multi-dimensional general relativistic neutrino hydrodynamics simulations of core-collapse supernovae. A comparison with models computed using either the purely Newtonian approximation or the "effective gravitational potential" approach reveals appreciable quantitative differences in the heating conditions and the gravitational wave spectra. Our results underscore the important role of general relativity in the supernova problem (which appears to be on par with other important factors such as the dimensionality and the equation of state) both for our understanding of the explosion dynamics as well as for predictions of observable signatures., Comment: Contribution to the proceedings of ASTRONUM-2011, 6 pages, 2 figures

We study the excitation of nonaxisymmetric modes in the post-merger phase of binary compact object mergers and the associated gravitational wave emission. Our analysis is based on general-relativistic simulations, in the spatial conformal flatness approximation, using smoothed-particle-hydrodynamics for the evolution of matter, and we use a set of equal and unequal mass models, described by two nonzero-temperature hadronic equations of state and by one strange star equation of state. Through Fourier transforms of the evolution of matter variables, we can identify a number of oscillation modes, as well as several nonlinear components (combination frequencies). We focus on the dominant m=2 mode, which forms a triplet with two nonlinear components that are the result of coupling to the quasiradial mode. A corresponding triplet of frequencies is identified in the gravitational wave spectrum, when the individual masses of the compact objects are in the most likely range of 1.2 to 1.35 $M_\odot$. We can thus associate, through direct analysis of the dynamics of the fluid, a specific frequency peak in the gravitational wave spectrum with the nonlinear component resulting from the difference between the m=2 mode and the quasiradial mode. Once such observations become available, both the m=2 and quasiradial mode frequencies could be extracted, allowing for the application of gravitational-wave asteroseismology to the post-merger remnant and leading to tight constraints on the equation of state of high-density matter., Comment: 11 pages, 8 figures, submitted to MNRAS

We present detailed results from performing general relativistic (GR) simulations of stellar core collapse to a proto-neutron star, using a microphysical equation of state (EoS) as well as an approximate description of deleptonization during the collapse phase. We show that for a wide variety of rotation rates and profiles the gravitational wave (GW) burst signals from the core bounce are of a generic type, already known as Type I in the literature. In addition, for most models the characteristic frequency of the GW burst signal lies in a narrow range around approximately 718 Hz. In our systematic study, using both GR and Newtonian gravity, we identify, individually quantify, and discuss in detail the micro- and macrophysical mechanisms leading to this result, i.e. the effects of rotation, the EoS, and deleptonization. We also discuss the detectability prospects of such GW burst signals by GW detectors, and infer that such a generic type of signal templates will likely facilitate a more efficient search in current and future detectors of both interferometric and resonant type., Comment: 10 pages, 9 figures, to appear in the Proceedings of the XLII Rencontres de Moriond, "Gravitational Waves and Experimental Gravity", La Thuile, Italy, 2007

We present the results of full 3D, long-term simulations of core-collapse supernova (CCSN) resulting from low-mass progenitors, covering the evolution from bounce through shock revival until shock breakout. We consider two low-mass iron-core progenitors which explode as CCSNe (one with solar metallicity and one with zero metallicity) and one progenitor with an oxygen-neon-magnesium (ONeMg) core which explodes as an ECSN. In the ECSN case mixing of heavy elements is inefficient because of a relatively spherical beginning of the explosion. The similarity between the core structure of the zero-metallicity progenitor and the ONeMg progenitor leads to similarities in their explosion properties and efficiency of mixing. In contrast, the solar-metallicity iron-core progenitor shows strong growth of Rayleigh-Taylor plumes in the entire helium core leading to more pronounced mixing and a remnant morphology similar to long-term simulations of more massive progenitors. Wir präsentieren 3D Langzeit Simulationen der Explosion von Kernkollaps-Supernovae vom “Bounce” über den Start der Explosion bis hin zum Schockausbruch, ausgehend von Vorgängern niedriger Masse. Wir nutzen zwei Vorgänger mit Eisenkernen (einen mit solarer Metallizität und einen mit “zero” Metallizität), die als Kernkollpas Supernova (CCSN) explodieren, und einen Vorgäner mit einem Sauerstoff-Neon-Magnesium (ONeMg) Kern, welcher als “Electron-Capture” Supernova (ECSN) explodiert. Im letzteren Fall is das Mischen schwerer Elemente nicht effizient da die Explosion bereits sphärisch beginnt. Die gleichartige Kernstruktur des “zero” Metallizität Vorgängers und des ONeMg Vorgängers führt zu sehr ähnlichen Eigenschaften der Explosion und des Mischverhaltens. Im Gegensatz dazu zeigt der Eisenkern Vorgänger solarer Metallizität starkes Wachstum von Rayleigh-Taylor Instabilitäten im Helium Kern, welche zu stärkerem Mischen und einer Morphologie führen, die Ähnlichkeiten mit der Morphologie schwererer Sterne haben.

In this thesis, we present a general relativistic multidimensional flux-limited diffusion scheme for neutrino transport. We apply the newly developed scheme to simulate core-collapse of very massive stars. We observe that in some of the models the stagnated shock is revived before the black hole formation. We continue the simulation beyond the black hole formation for one model. We observe that the shock continues to expand until the end of the simulation. In dieser Arbeit stellen wir ein allgemein relativistisches, mehrdimensionales, flussbegrenztes Diffusionsschema für den Neutrinotransport vor. Wir verwenden das neu entwickelte Schema, um den Kernkollaps von sehr massenreichen Sternen zu simulieren. Wir beobachten, dass in einigen der Sternmodelle der stagnierte Schock vor der Bildung des schwarzen Loches wieder auflebt. Fuer einen Fall, setzen wir die Simulation jenseits der Bildung des schwarzen Loches fort und beobachten, dass sich der Schock bis zum Ende der Simulation weiter ausdehnt.

A challenging problem in core-collapse supernova (CCSN) theory is the modeling of neutrino transport in self-consistent simulations. In this thesis, we assess the influence of two different neutrino-transport methods on the delayed neutrino-heating mechanism of CCSNe. To this end, we compare self-consistent simulations in three spatial dimensions with either "ray-by-ray-plus" or fully multidimensional neutrino transport. Ein zentrales Problem in der Theorie von Kernkollaps-Supernovae (KKSNe) ist die Modellierung des Neutrinotransports in selbstkonsistenten Simulationen. In dieser Arbeit wird der Einfluss zweier Neutrinotransport-Methoden auf den verzögerten Neutrino-Heizmechanismus von KKSNe untersucht. Hierzu werden selbstkonsistente Simulationen in drei Raumdimensionen verglichen, die entweder mit der "Ray-By-Ray-Plus"-Näherung oder mit vollständig mehrdimensionalem Neutrinotransport gerechnet wurden.

Protoneutron stars (PNS) formed in core-collapse supernovae (CCSNe) may contain an abundant number of muons due to the high temperatures and the high electron chemical potentials. The appearance of muons “softens” the PNS equation of state, triggers faster PNS contraction, and thus leads to increased luminosities and mean energies of all emitted neutrinos. In this thesis, I present the first self-consistent implementation of muons into CCSN simulations and their effect on the delayed neutrino-driven explosion mechanism. Die hohen Temperaturen und chemischen Potentiale der Elektronen in den bei Kernkollaps-Supernovae (KKSN) auftretenden Protoneutronen Sternen erlauben eine große Zahl von Myonen im dichten Medium. Ihr Auftreten macht die Zustandsgleichung “weicher”, was zur schnelleren Kontraktion und damit zu erhöhten Luminositäten und mittleren Energien aller emittierten Neutrinos führt. In dieser Dissertation bespreche ich die erste selbstkonsistente Implementierung von Myonen in KKSN Simulationen und ihre Auswirkungen auf den neutrino-getriebenen Explosionsmechanismus.

The material ejected from the collision (merger) of two neutron stars has been predicted as the origin of elements heavier than iron. We study the impact of neutrino emission and absorption on the composition of the ejecta in numerical simulations of neutron star mergers. For this purpose, we introduce a new treatment of the neutrino effects in mergers, ILEAS (Improved Leakage-Equilibration-Absorption Scheme), and demonstrate its performance in astrophysical test scenarios and applications. Theoretische Modelle sagen vorher, dass Elemente schwerer als Eisen in der ausgeschleuderten Materie bei Neutronenstern-Verschmelzungen produziert werden. Wir untersuchen mit Hilfe von numerischen Simulationen den Einfluss von Neutrino-emission und -absorption auf die Zusammensetzung des ausgestoßenen Materials. Zu diesem Zweck entwickeln wir ein neues Modell für Neutrinoeffekte in Neutronenstern-Verschmelzungen, ILEAS (``Improved Leakage-Equilibration-Absorption Scheme''), und demonstrieren seine Leistungsfahigkeit in astrophysikalischen Testszenarios.

Neutrino-hydrodynamics simulations of 767 massive-star models were conducted to explore the progenitor-SN systematics. SN 1987A and the Crab SN were the observational anchors of this theoretical survey. Two parameters were identified of the pre-SN structure based on the "critical luminosity" concept allowing for a clear separation of explosions and failed SNe. Nucleosynthesis, light curves, explosion energies, and remnant masses were determined and compared to the population of observed SNe. Es wurden Neutrino-Hydrodynamik-Simulationen von 767 Modellen massereicher Sterne durchgeführt, um die Vorläufer-Supernova-Systematik zu erforschen. SN 1987A und die Krebs-SN waren die Beobachtungsankerpunkte dieser theoretischen Studie. Zwei Strukturparameter des Prä-SN-Modells wurden, basierend auf dem "Kritische-Luminositäts- Konzept", bestimmt, die Explosionen und gescheiterte SNe klar separieren. Nukleosynthese, Lichtkurven, Explosionsenergien und SN-Überrest-Massen wurden zum Vergleich mit der beobachteten SN-Population bestimmt.

Core-collapse supernovae are tremendous explosions of massive stars. Their explosion mechanism is matter of intense research and can only be studied in detail with the help of computer simulations. In this thesis, we present the first successfully exploding three-dimensional core-collapse supernova simulations performed with sophisticated neutrino transport. Comparisons to axially symmetric models and non-exploding cases reveal how hydrodynamic instabilities and neutrino interactions affect the explosion process. Kernkollaps-Supernovae sind gewaltige Explosionen massereicher Sterne. Ihr Explosionsmechanismus ist Gegenstand intensiver Forschung und kann nur mittels Computersimulationen im Detail untersucht werden. In dieser Arbeit werden die ersten erfolgreich explodierenden dreidimensionalen Simulationen von Kernkollaps-Supernovae beschrieben, die mit hochentwickeltem Neutrinotransport durchgeführt wurden. Vergleiche zu axialsymmetrischen Modellen und fehlgeschlagenen Explosionen zeigen auf, wie hydrodynamische Instabilitäten und Neutrino-Wechselwirkungen den Explosionsprozess beeinflussen.

This work is concerned with the formation of the chemical elements during core-collapse supernovae (SN) explosions. I have investigated the role played by possible eV-mass sterile neutrinos and self-interactive potentials of nucleons on the nucleosynthesis in neutrino-driven winds of proto-neutron stars. I also investigated the asymmetric formation of 44Ti and 56Ni as diagnostically relevant radioactive species in new three-dimensional SN explosion models. In der vorliegenden Arbeit habe ich mich mit der Erzeugung von chemischen Elementen in Kern-Kollaps Supernovae (SN) beschaeftigt. Ich habe die Rolle untersucht, welche hypothetische sterile Neutrinos mit eV Masse und die Selbstwechselwirkung Potenziale von Nukleonen bei der Nukleosynthese im neutrinogetriebenen Wind von Proto-Neutronensternen spielen. Weiterhin habe ich die Erzeugung von 44Ti und 56Ni in neuen dreidimensionalen Explosionsmodellen von Supernovae untersucht.

Electron capture supernovae are explosions of low-mass, Oxygen-Neon-Magnesium core stars. These complex events have to be studied with the help of numerical simulations. In this work we use general relativistic, multi dimensional simulations to investigate the resulting neutrino signal, and the properties of the ejected matter. We compare a large number of simulations with different neutron star equations of state and neutrino rate prescriptions, and make predictions on observable quantities. Electron-Capture Supernovae sind Explosionen von leichten Sternen mit Sauerstoff-Neon-Magnesium Kernen. Diese komplizierten Vorgänge müssen mit numerischen Simulationen untersucht werden. Wir benutzen allgemein-relativistische, mehrdimensionale Simulationen, um das entstehende Neutrinosignal und die Eigenschaften der ausgeschleuderten Materie zu untersuchen. Dazu machen wir Vorhersagen für eine große Anzahl von Simulationen mit verschiedenen Neutronensternzustandsgleichungen und Neutrinoraten.

When a massive star dies by gravitational collapse a neutron star forms, which radiates a large amount of neutrinos. In my thesis, this neutrino signal from neutron star and black hole formation is calculated by hydrodynamical modelling of the stellar collapse, and astrophysical implications are explored. Beim Tod eines massereichen Sterns entsteht ein Neutronenstern, der große Mengen an Neutrinos abstrahlt. In meiner Arbeit wird dieses Neutrinosignal der Neutronenstern- und Schwarze-Loch-Bildung durch hydrodynamische Modellierung des Sternkollapses berechnet und astrophysikalische Konsequenzen werden untersucht.

Core-collapse supernova explosions are the catastrophic death of stars more massive than about eight solar masses. To make progress towards a final understanding of the explosion mechanism, one heavily rely on computer simulations. In this thesis, we present the very first 3D simulations of core-collapse supernova explosions with fully self-consistent neutrino transport along with a large set of successfully exploding 2D models. Kernkollaps-Supernova Explosionen sind der spektakuläre Tod von Sternen, die schwerer als etwa acht Sonnenmassen sind. Um einem vollständigen Verständnis der Vorgänge in Supernovae näher zu kommen, ist man stark auf numerische Simulationen angewiesen. Die vorliegende Arbeit präsentiert die allerersten 3D Simulationen von Kernkollapse-Supernova Explosionen mit detailliertem Neutrinotransport, sowie eine größere Zusammenstellung an erfolgreichen Explosionsmodellen in zwei Dimensionen.

We present the results of spherically symmetric explosion simulations for a set of about 100 progenitor stars of solar metallicity, performed with the aim of better understanding the connection between progenitors and the properties of core-collapse supernovae. The explosions were initiated by means of a neutrino-heating scheme that depends on parametrized neutrino quantities (luminosities and mean spectral energies) based on an analytic cooling model of the nascent neutron star. The evolution of the models was followed beyond shock breakout from the stellar surface. Because of strong star-to-star variations of the progenitor structure at the onset of collapse, we find a great variability of supernova properties (such as explosion energy, compact remnant mass, and ejected nickel mass) even in narrow progenitor-mass windows. Our novel approach and results allow to establish better links of theoretical explosion models with observations. Wir präsentieren die Ergebnisse kugelsymmetrischer Explosionsrechnungen für über 100 Vorläufersternen mit solarer Metallizität, durchgeführt mit dem Ziel, den Zusammenhang zwischen den Vorläufersternen und den Eigenschaften von Kernkollaps-Supernovae besser zu verstehen. Die Explosionen wurden eingeleitet mittels eines Neutrino-Heizschemas, dessen parametrisierte Neutrino-Größen (Leuchtstärken und mittlere spektrale Energien) aus einem analytischen Kühlmodell für den Neutronenstern abgeleitet wurden. Die Modelle wurden bis über den Stoßausbruch aus der Sternoberfläche hinaus entwickelt. Aufgrund erheblicher Stern-zu-Stern-Variationen der Vorläuferstruktur vor dem stellaren Kollaps finden wir starke Unterschiede der Supernova-Eigenschaften (z. B. der Explosionsenergie, Masse des kompakten Überrestes und Masse des ausgestoßenen Nickels) sogar in kleinen Masseintervallen der Vorläufersterne. Unsere neuartigen Ergebnisse öffnen den Weg zu einer besseren Verknüpfung von theoretischen Supernovamodellen und Beobachtungen.

By means of computer simulations we investigate models for the system of a black-hole accretion disk as a possible remnant of the merger of a neutron star with a neutron star or a black hole. Using an approximate description for multidimensional neutrino transport, we calculate the nucleosynthesis relevant properties of the matter outflows and we evaluate the relevance of neutrinos for the initiation of a short gamma-ray burst. Anhand von Computersimulationen werden Modelle für das System einer Akkretionsscheibe um ein Schwarzes Loch untersucht, das bei einer Verschmelzung von einem Neutronenstern mit einem Neutronenstern oder einem Schwarzen Loch entstehen kann. Mithilfe einer approximativen Behandlung von multidimensionalem Neutrinotransport werden die Nukleosynthese-relevanten Eigenschaften der Materieausflüsse berechnet und die Relevanz von Neutrinos für das Einleiten eines kurzen Gammastrahlenblitzes ausgewertet.

Under the assumption that the energy of the ground state of 3-flavor quark matter is lower than the one of nucleonic matter, the compact stellar remnants of supernova explosions are composed of this quark matter. Because of the appearence of strange quarks, such objects are called strange stars. Considering their observational features, strange stars are very similar to neutron stars made of nucleonic matter, and therefore observations cannot exclude the existence of strange stars. This thesis introduces a new method for simulating mergers of compact stars and black holes within a general relativistic framework. The main goal of the present work is the investigation of the question, whether the coalescence of two strange stars in a binary system yields observational signatures that allow one to distinguish them from colliding neutron stars. In this context the gravitational-wave signals are analyzed. It is found that in general the characteristic frequencies in the gravitational-wave spectra are higher for strange stars. Moreover, the amount of matter that becomes gravitationally unbound during the merging is determined. The detection of ejecta of strange star mergers as potential component of cosmic ray flux could serve as a proof of the existence of strange quark matter. Unter der Annahme, dass die Grundzustandsenergie von 3-flavor-Quarkmaterie niedriger als die von nukleonischer Materie ist, bestehen die kompakten stellaren Überreste von Supernova-Explosionen aus dieser Quarkmaterie und werden aufgrund der auftretenden strange-Quarks Seltsame Sterne genannt. Diese Objekte ähneln in ihren Eigenschaften sehr stark Neutronensternen aus nukleonischer Materie, weshalb Beobachtungen die Existenz von Seltsamen Sternen bisher nicht ausschliessen können. Neben der Entwicklung einer neuen Methode zur Simulation von Kollisionen von kompakten Sternen mit Schwarzen Löchern im Rahmen der Einsteinschen Allgemeinen Relativitätstheorie hat sich die vorliegende Arbeit hauptsächlich mit der Frage beschäftigt, ob die Verschmelzung zweier Seltsamer Sterne in einem Doppelsternsystem zu observablen Signaturen führt, die sie grundsätzlich von verschmelzenden Neutronensternen unterscheiden. In diesem Zusammenhang wird das Gravitationswellensignal analysiert, wobei sich zeigt, dass charakteristische Frequenzen des Gravitationswellenspektrums für Seltsame Sterne im Allgemeinen bei höheren Werten liegen. Weiterhin wird bestimmt, wie viel Quarkmaterie während eines Verschmelzungsprozesses gravitativ ungebunden wird, und in diesem Fall als eindeutig messbare Komponente in der Kosmischen Strahlung zum Nachweis der Existenz seltsamer Materie dienen kann.

Hydrodynamic instabilities play an important role in the neutrino-driven supernova explosions of massive stars. On the one hand it has been shown that these instabilities enhance the efficiency of neutrino heating, which seems to be required to allow for explosions, and on the other hand they provide a possible explanation for the pronounced explosion anisotropy found in observations. In this work the nature of the instabilities and their impact on the explosion is studied by means of hydrodynamic simulations. The anisotropy is caused by a newly discovered instability, which is based on the coupling of sound waves and vorticity. Furthermore, the simulations show that the anisotropy of the ejected matter leads to a recoil of the neutron star that is formed in the supernova explosion. This mechanism can explain the observed large neutron star velocities, in some cases more than 1000 km/s. Hydrodynamische Instabilitäten spielen eine wichtige Rolle für die neutrinogetriebene Supernovaexplosion massereicher Sterne. Einerseits wurde gezeigt, dass sie die Effizienz des Neutrinoheizens erhöhen und damit derartige Explosionen überhaupt erst ermöglichen könnten, und andererseits könnten sie eine Erklärung für die beobachtete starke Anisotropie der Explosionen darstellen. In dieser Arbeit wird die Natur der Instabilitäten und ihre Auswirkungen auf die Explosion mit Hilfe hydrodynamischer Simulationen untersucht. Die Anisotropie wird durch einen neu entdeckte Instabilität verursacht, die auf der Kopplung von Schallwellen und Vortizität basiert. Weiterhin wird gezeigt, dass die starke Anisotropie der weggeschleuderten Materie zu einem Rückstoß des in der Supernova gebildeten Neutronensterns führt, der die beoachteten extremen Geschwindigkeiten dieser Objekte, in einigen Fällen mehr als 1000 km/s, erklären kann.

Core Collapse supernovae are among the most energetic events which are observed in our Universe. Only with numerical simulations it is possible to investigate the complex events that lead to an explosion. The details of the explosion mechanism of core collapse supernovae is yet not understood, which to some extend is due to the fact that the equation of state, which describes the matter inside the star, is poorly known. In this study the worldwide first simulations of core collapse supernova are discussed, which investigate in multi--dimensional simulations the influence of the equation of state on the explosion mechanism. It was shown that the equation of state strongly influences the supernova evolution and that thus a better knowledge of the equation of state is crucial for the understanding of the explosion mechanism. Kernkollapse-Supernovae gehören mit zu den energiereichsten Explosionen die man in unserem Universum beobachten kann. Nur mittels numerischer Simulationen ist es möglich, die komplexen Vorgänge, die zu diesen Explosionen führen, zu verstehen. Der genaue Mechanismus für diese Explosionen ist bis heute noch nicht verstanden, was unter anderem daran liegt, dass die Zustandsgleichung für die Beschreibung der Materie im Sterninneren nur unzureichend bekannt ist. Die vorliegende Arbeit präsentiert die weltweit ersten Supernovasimulationen in denen mittels mehrdimensionaler Rechnungen der Einfluss der Zustandsgleichung auf den Explosionsmechanismus untersucht werden konnnte. Es konnte gezeigt werden, dass die Zustandsgleichung einen erheblichen Einfluss auf die Vorgänge innerhalb der Supernova hat, und dass deshalb eine bessere Kenntnis der Zustandsgleichung zum Verständnis von Kernkollaps-Supernovae unerlässlich ist.

The elements heavier than iron are produced by neutron capture under extreme conditions. In core-collapse supernovae, which are the final stage of the live of massive stars, such conditions can be fulfil. The hydrodynamical simulations presented in this work follow the evolution of the explosion and the subsequent neutrino-driven wind showing, for the first time in detail, how the interaction of the wind with the slow-moving ejecta leads to a wind termination shock, which is progenitor and time dependent. The change in the behavior of the matter, when is decelerated at the shock, can influence the nucleosynthesis yields. Detailed nuclosynthesis network calculations based on the dynamical results of this work are required to come to more definitive predictions about the production of heavy elements. Die Elemente schwerer als Eisen werden durch Neutroneneinfang unter extremen Bedingungen erzeugt. In Gravitationskollaps-Supernovae, die das Leben massereicher Sterne beenden, können diese Bedingungen erreicht werden. Die in dieser Arbeit vorgestellten hydrodynamischen Simulationen zur Entwicklung dieser Stern-Explosionen und des dabei auftretenden neutrinogetriebenen Windes zeigen erstmals im Detail, wie bei Wechselwirkung dieses Windes mit der früher ausgestoßenen Materie, die sich langsamer nach außen bewegt, eine Stoßwelle entsteht, der Verhalten vom Vorläuferstern abhängt und zeitabhängig ist. Die Veränderung der Bedingungen in der Materie, die durch diesen Stoß abgebremst wird, kann einen großen Einfluß auf die Nukleosynthese haben. Eine genauere Untersuchung solcher Auswirkungen erfordert weitere Rechnungen mit detaillierten Brennnetzwerken, basierend auf den dynamischen Ergebnissen dieser Arbeit.

Core collapse supernovae are spectacular phenomena, which have proven to be interesting in several aspects, including their significant contribution to the nucleosynthesis of heavy elements and the production of measurable neutrino signals and gravitational waves. In order to understand the extreme complexity of supernovae it is absolutely necessary to perform numerical simulations. In this work I present the worldwide first supernova simulations which take into account two effects which are crucial for the explosion mechanism: The exact treatment of spectral neutrino transport and of neutrino interactions and multi-dimensional hydrodynamic instabilities. A new type of instability was identified to be important for the explosion mechanism. It is necessary to perform further simulations to verify this. Kernkollaps-Supernovae sind extrem interessante und spektakuläre Phänomene, zum Einen wegen ihres Beitrags zur Nukleosynthese schwerer Elemente, zum Anderen wegen der Erzeugung messbarer Neutrino- und Gravitationswellensignale. Numerische Simulationen sind zum Verständnis der extrem komplexen Vorgänge in Supernovae unerlässlich. Die vorliegende Arbeit präsentiert die weltweit ersten Supernovarechnungen in denen zwei für den Explosionsmechanismus wichtige Effekte in bisher unerreichter Genauigkeit berücksichtigt werden konnten: Der Transport von Neutrinos und ihre Wechselwirkung mit dem Medium des Sterns, sowie hydrodynamische Instabilitäten. Ein neuer Typ von Instabilität konnte identifiziert werden, der in unseren Simulationen Explosionen begünstigt und wichtige Konsequenzen für den Explosionsmechanismus hat. Weitere Rechnungen sind notwendig um diese Aussage zu verifizieren.