arXiv subjects

Andreas Marek

Publications and source records attributed to Andreas Marek.

29 records · Page 2Linked to original sources

arXiv2012

Core-Collapse Supernovae: Reflections and Directions

Core-collapse supernovae are among the most fascinating phenomena in astrophysics and provide a formidable challenge for theoretical investigation. They mark the spectacular end of the lives of massive stars and, in an explosive eruption, release as much energy as the sun produces during its whole life. A better understanding of the astrophysical role of supernovae as birth sites of neutron stars, black holes, and heavy chemical elements, and more reliable predictions of the observable signals from stellar death events are tightly linked to the solution of the long-standing puzzle how collapsing stars achieve to explode. In this article our current knowledge of the processes that contribute to the success of the explosion mechanism are concisely reviewed. After a short overview of the sequence of stages of stellar core-collapse events, the general properties of the progenitor-dependent neutrino emission will be briefly described. Applying sophisticated neutrino transport in axisymmetric (2D) simulations with general relativity as well as in simulations with an approximate treatment of relativistic effects, we could find successful neutrino-driven explosions for a growing set of progenitor stars. First results of three-dimensional (3D) models have been obtained, and magnetohydrodynamic simulations demonstrate that strong initial magnetic fields in the pre-collapse core can foster the onset of neutrino-powered supernova explosions even in nonrotating stars. These results are discussed in the context of the present controversy about the value of 2D simulations for exploring the supernova mechanism in realistic 3D environments, and they are interpreted against the background of the current disagreement on the question whether the standing accretion shock instability (SASI) or neutrino-driven convection is the crucial agency that supports the onset of the explosion.

astro-ph.SR↗

arXiv2012

Progenitor-Explosion Connection and Remnant Birth Masses for Neutrino-Driven Supernovae of Iron-Core Progenitors

We perform hydrodynamic supernova simulations in spherical symmetry for over 100 single stars of solar metallicity to explore the progenitor-explosion and progenitor-remnant connections established by the neutrino-driven mechanism. We use an approximative treatment of neutrino transport and replace the high-density interior of the neutron star (NS) by an inner boundary condition based on an analytic proto-NS core-cooling model, whose free parameters are chosen such that explosion energy, nickel production, and energy release by the compact remnant of progenitors around 20 solar masses are compatible with Supernova 1987A. Thus we are able to simulate the accretion phase, initiation of the explosion, subsequent neutrino-driven wind phase for 15-20 s, and the further evolution of the blast wave for hours to days until fallback is completed. Our results challenge long-standing paradigms. We find that remnant mass, launch time, and properties of the explosion depend strongly on the stellar structure and exhibit large variability even in narrow intervals of the progenitors' zero-age-main-sequence mass. While all progenitors with masses below about 15 solar masses yield NSs, black hole (BH) as well as NS formation is possible for more massive stars, where partial loss of the hydrogen envelope leads to weak reverse shocks and weak fallback. Our NS baryonic masses of ~1.2-2.0 solar masses and BH masses >6 solar masses are compatible with a possible lack of low-mass BHs in the empirical distribution. Neutrino heating accounts for SN energies between some 10^{50} erg and about 2*10^{51} erg, but can hardly explain more energetic explosions and nickel masses higher than 0.1-0.2 solar masses. These seem to require an alternative SN mechanism.

astro-ph.SR↗

arXiv2012

Is Strong SASI Activity the Key to Successful Neutrino-Driven Supernova Explosions?

Following a simulation approach of recent publications we explore the viability of the neutrino-heating explosion mechanism in dependence on the spatial dimension. Our results disagree with previous findings. While we also observe that two-dimensional (2D) models explode for lower driving neutrino luminosity than spherically symmetric (1D) models, we do not find that explosions in 3D occur easier and earlier than in 2D. Moreover, we find that the average entropy of matter in the gain layer hardly depends on the dimension and thus is no good diagnostic quantity for the readiness to explode. Instead, mass, integrated entropy, total neutrino-heating rate, and nonradial kinetic energy in the gain layer are higher when models are closer to explosion. Coherent, large-scale mass motions as typically associated with the standing accretion-shock instability (SASI) are observed to be supportive for explosions because they drive strong shock expansion and thus enlarge the gain layer. While 2D models with better angular resolution explode clearly more easily, the opposite trend is seen in 3D. We interpret this as a consequence of the turbulent energy cascade, which transports energy from small to large spatial scales in 2D, thus fostering SASI activity. In contrast, the energy flow in 3D is in the opposite direction, feeding fragmentation and vortex motions on smaller scales and thus making the 3D evolution with finer grid resolution more similar to 1D. More favorable conditions for explosions in 3D may therefore be tightly linked to efficient growth of low-order SASI modes including nonaxisymmetric ones.

astro-ph.SR↗

arXiv2011

Core-Collapse Supernovae: Explosion Dynamics, Neutrinos and Gravitational Waves

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 11.2 and a 15 solar mass progenitor. We also mention 3D effects as another aspect in supernova physics awaiting further, more thorough investigation.

astro-ph.SR↗

arXiv2011

General Relativistic Explosion Models of Core-Collapse Supernovae

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.

astro-ph.SR↗

arXiv2010

Fast time variations of supernova neutrino fluxes and their detectability

In the delayed explosion scenario of core-collapse supernovae (SNe), the accretion phase shows pronounced convective overturns and a low-multipole hydrodynamic instability, the standing accretion shock instability (SASI). These effects imprint detectable fast time variations on the emerging neutrino flux. Among existing detectors, IceCube is best suited to this task, providing an event rate of ~1000 events per ms during the accretion phase for a fiducial SN distance of 10 kpc, comparable to what could be achieved with a megaton water Cherenkov detector. If the SASI activity lasts for several hundred ms, a Fourier component with an amplitude of 1% of the average signal clearly sticks out from the shot noise. We analyze in detail the output of axially symmetric hydrodynamical simulations that predict much larger amplitudes up to frequencies of a few hundred Hz. If these models are roughly representative for realistic SNe, fast time variations of the neutrino signal are easily detectable in IceCube or future megaton-class instruments. We also discuss the information that could be deduced from such a measurement about the physics in the SN core and the explosion mechanism of the SN.

astro-ph.HE↗

arXiv2008

The gravitational wave burst signal from core collapse of rotating stars

We present results from detailed general relativistic simulations of stellar core collapse to a proto-neutron star, using two different microphysical nonzero-temperature nuclear equations of state as well as an approximate description of deleptonization during the collapse phase. Investigating a wide variety of rotation rates and profiles as well as masses of the progenitor stars and both equations of state, we confirm in this very general setup the recent finding that a generic gravitational wave burst signal is associated with core bounce, already known as type I in the literature. The previously suggested type II (or "multiple-bounce") waveform morphology does not occur. Despite this reduction to a single waveform type, we demonstrate that it is still possible to constrain the progenitor and postbounce rotation based on a combination of the maximum signal amplitude and the peak frequency of the emitted gravitational wave burst. Our models include to sufficient accuracy the currently known necessary physics for the collapse and bounce phase of core-collapse supernovae, yielding accurate and reliable gravitational wave signal templates for gravitational wave data analysis. In addition, we assess the possiblity of nonaxisymmetric instabilities in rotating nascent proto-neutron stars. We find strong evidence that in an iron core-collapse event the postbounce core cannot reach sufficiently rapid rotation to become subject to a classical bar-mode instability. However, many of our postbounce core models exhibit sufficiently rapid and differential rotation to become subject to the recently discovered dynamical instability at low rotation rates.

astro-ph↗

arXiv2007

Generic Gravitational Wave Signals from the Collapse of Rotating Stellar Cores

We perform general relativistic simulations of stellar core collapse to a proto-neutron star, using a microphysical equation of state as well as an approximate description of deleptonization. We show that for a wide variety of rotation rates and profiles the gravitational wave burst signals from the core bounce are of a generic type, known as Type I in the literature. In our systematic study, using both general relativity and Newtonian gravity, we identify and individually quantify the micro- and macrophysical mechanisms leading to this result, i.e. the effects of rotation, the equation of state, and deleptonization. Such a generic type of signal templates will likely facilitate a more efficient search in current and future gravitational wave detectors of both interferometric and resonant type.

astro-ph↗

arXiv2007

Generic Gravitational Wave Signals from the Collapse of Rotating Stellar Cores: A Detailed Analysis

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.

astro-ph↗

arXiv2007

Rotating Collapse of Stellar Iron Cores in General Relativity

We present results from the first 2+1 and 3+1 simulations of the collapse of rotating stellar iron cores in general relativity employing a finite-temperature equation of state and an approximate treatment of deleptonization during collapse. We compare full 3+1 and conformally-flat spacetime evolution methods and find that the conformally-flat treatment is sufficiently accurate for the core-collapse supernova problem. We focus on the gravitational wave (GW) emission from rotating collapse, core bounce, and early postbounce phases. Our results indicate that the GW signature of these phases is much more generic than previously estimated. In addition, we track the growth of a nonaxisymmetric instability of dominant m = 1 character in two of our models that leads to prolonged narrow-band GW emission at ~ 920-930 Hz over several tens of milliseconds.

astro-ph↗

arXiv2006

New methods for approximating general relativity in numerical simulations of stellar core collapse

We review various approaches to approximating general relativistic effects in hydrodynamic simulations of stellar core collapse and post-bounce evolution. Different formulations of a modified Newtonian gravitational potential are presented. Such an effective relativistic potential can be used in an otherwise standard Newtonian hydrodynamic code. An alternative approximation of general relativity is the assumption of conformal flatness for the three-metric, and its extension by adding second post-Newtonian order terms. Using a code which evolves the coupled system of metric and fluid equations, we apply the various approximation methods to numerically simulate axisymmetric models for the collapse of rotating massive stellar cores. We compare the collapse dynamics and gravitational wave signals (which are extracted using the quadrupole formula), and thereby assess the quality of the individual approximation method. It is shown that while the use of an effective relativistic potential already poses a significant improvement compared to a genuinely Newtonian approach, the two conformal-flatness-based approximation methods yield even more accurate results, which are qualitatively and quantitatively very close to those of a fully general relativistic code even for rotating models which almost collapse to a black hole.

astro-ph↗