arXiv subjects

H. -Thomas Janka

Publications and source records attributed to H. -Thomas Janka.

51 records · Page 3Linked to original sources

arXiv2013

New fission fragment distributions and r-process origin of the rare-earth elements

Neutron star (NS) merger ejecta offer a viable site for the production of heavy r-process elements with nuclear mass numbers A > 140. The crucial role of fission recycling is responsible for the robustness of this site against many astrophysical uncertainties, but calculations sensitively depend on nuclear physics. In particular the fission fragment yields determine the creation of 110 < A < 170 nuclei. Here we apply a new scission-point model, called SPY, to derive the fission fragment distribution (FFD) of all relevant neutron-rich, fissioning nuclei. The model predicts a doubly asymmetric FFD in the abundant A ~ 278 mass region that is responsible for the final recycling of the fissioning material. Using ejecta conditions based on relativistic NS merger calculations we show that this specific FFD leads to a production of the A ~ 165 rare-earth peak that is nicely compatible with the abundance patterns in the Sun and metal-poor stars. This new finding further strengthens the case of NS mergers as possible dominant origin of r-nuclei with A > 140.

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arXiv2013

Natal Kicks of Stellar-Mass Black Holes by Asymmetric Mass Ejection in Fallback Supernovae

Integrating trajectories of low-mass X-ray binaries containing black holes within the Galactic potential, Repetto, Davies & Sigurdsson recently showed that the large distances of some systems above the Galactic plane can only be explained if black holes receive appreciable natal kicks. Surprisingly, they found that the distribution of black hole kick velocities (rather than that of the momenta) should be similar to that of neutron stars. Here I argue that this result can be understood if neutron star and black hole kicks are a consequence of large-scale asymmetries created in the supernova ejecta by the explosion mechanism. The corresponding anisotropic gravitational attraction of the asymmetrically expelled matter does not only accelerate new-born neutron stars by the "gravitational tug-boat mechanism". It can also lead to delayed black-hole formation by asymmetric fallback of the slowest parts of the initial ejecta onto the transiently existing neutron star, in course of which the momentum of the black hole can grow with the fallback mass. Black hole kick velocities will therefore not be reduced by the ratio of neutron star to black hole mass as would be expected for kicks caused by anisotropic neutrino emission of the nascent neutron star.

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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.

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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.

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arXiv2012

Explosion Mechanisms of Core-Collapse Supernovae

Supernova theory, numerical and analytic, has made remarkable progress in the past decade. This progress was made possible by more sophisticated simulation tools, especially for neutrino transport, improved microphysics, and deeper insights into the role of hydrodynamic instabilities. Violent, large-scale nonradial mass motions are generic in supernova cores. The neutrino-heating mechanism, aided by nonradial flows, drives explosions, albeit low-energy ones, of ONeMg-core and some Fe-core progenitors. The characteristics of the neutrino emission from new-born neutron stars were revised, new features of the gravitational-wave signals were discovered, our notion of supernova nucleosynthesis was shattered, and our understanding of pulsar kicks and explosion asymmetries was significantly improved. But simulations also suggest that neutrino-powered explosions might not explain the most energetic supernovae and hypernovae, which seem to demand magnetorotational driving. Now that modeling is being advanced from two to three dimensions, more realism, new perspectives, and hopefully answers to long-standing questions are coming into reach.

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arXiv2011

R-Process Nucleosynthesis in Dynamically Ejected Matter of Neutron Star Mergers

Although the rapid neutron-capture process, or r-process, is fundamentally important for explaining the origin of approximately half of the stable nuclei with A > 60, the astrophysical site of this process has not been identified yet. Here we study r-process nucleosynthesis in material that is dynamically ejected by tidal and pressure forces during the merging of binary neutron stars (NSs) and within milliseconds afterwards. For the first time we make use of relativistic hydrodynamical simulations of such events, defining consistently the conditions that determine the nucleosynthesis, i.e., neutron enrichment, entropy, early density evolution and thus expansion timescale, and ejecta mass. We find that 10^{-3}-10^{-2} solar masses are ejected, which is enough for mergers to be the main source of heavy (A > 140) galactic r-nuclei for merger rates of some 10^{-5} per year. While asymmetric mergers eject 2-3 times more mass than symmetric ones, the exact amount depends weakly on whether the NSs have radii of ~15 km for a "stiff" nuclear equation of state (EOS) or ~12 km for a "soft" EOS. R-process nucleosynthesis during the decompression becomes largely insensitive to the detailed conditions because of efficient fission recycling, producing a composition that closely follows the solar r-abundance distribution for nuclei with mass numbers A > 140. Estimating the light curve powered by the radioactive decay heating of r-process nuclei with an approximative model, we expect high emission in the B-V-R bands for 1-2 days with potentially observable longer duration in the case of asymmetric mergers because of the larger ejecta mass.

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arXiv2003

Towards Gravitational Wave Signals from Realistic Core Collapse Supernova Models

We have computed the gravitational wave signal from supernova core collapse using the presently most realistic input physics available. We start from state-of-the-art progenitor models of rotating and non-rotating massive stars, and simulate the dynamics of their core collapse by integrating the equations of axisymmetric hydrodynamics together with the Boltzmann equation for the neutrino transport including an elaborate description of neutrino interactions, and a realistic equation of state. We compute the quadrupole wave amplitudes, the Fourier wave spectra, the amount of energy radiated in form of gravitational waves, and the S/N ratios for the LIGO and the tuned Advanced LIGO interferometers resulting both from non-radial mass motion and anisotropic neutrino emission. The simulations demonstrate that the dominant contribution to the gravitational wave signal is produced by neutrino-driven convection behind the supernova shock. For stellar cores rotating at the extreme of current stellar evolution predictions, the core-bounce signal is detectable with advanced LIGO up to a distance of 5kpc, whereas the signal from post-shock convection is observable up to a distance of about 100kpc. If the core is non-rotating its gravitational wave emission can be measured up to a distance of 15kpc, while the signal from the Ledoux convection in the deleptonizing, nascent neutron star can be detected up to a distance of 10kpc. Both kinds of signals are generically produced by convection in any core collapse supernova.

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arXiv2002

Radiation hydrodynamics with neutrinos: Variable Eddington factor method for core-collapse supernova simulations

Neutrino transport and neutrino interactions in dense matter play a crucial role in stellar core collapse, supernova explosions and neutron star formation. Here we present a detailed description of a new numerical code for treating the time and energy dependent neutrino transport in hydrodynamical simulations of such events. The code is based on a variable Eddington factor method to deal with the integro-differential character of the Boltzmann equation. The moments of the neutrino distribution function and the energy and lepton number exchange with the stellar medium are determined by iteratively solving the zeroth and first order moment equations in combination with a model Boltzmann equation. The latter is discretized on a grid of tangent rays. The integration of the transport equations and the neutrino source terms is performed in a time-implicit way. In the present version of the program, the transport part is coupled to an explicit hydrodynamics code which follows the evolution of the stellar plasma by a finite-volume method with piecewise parabolic interpolation, using a Riemann solver for calculating the hydrodynamic states. The neutrino source terms are implemented in an operator-split step. Neutrino transport and hydrodynamics can be calculated with different spatial grids and different time steppings. The structure of the described code is modular and offers a high degree of flexibility for an application to relativistic and multi-dimensional problems at different levels of refinement and accuracy. We critically evaluate results for a number of test cases, including neutrino transport in rapidly moving stellar media and approximate relativistic core collapse, and suggest a path for generalizing the code to be used in multi-dimensional simulations of convection in neutron stars and supernovae.

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arXiv1998

Colliding neutron stars --- Gravitational waves, neutrino emission, and gamma-ray bursts

Three-dimensional hydrodynamical simulations are presented for the direct head-on or off-center collision of two neutron stars, employing a basically Newtonian PPM code but including the emission of gravitational waves and their back-reaction on the hydrodynamical flow. A physical nuclear equation of state is used that allows us to follow the thermodynamical evolution of the stellar matter and to compute the emission of neutrinos. Predicted gravitational wave signals, luminosities and waveforms, are presented. The models are evaluated for their implications for gamma-ray burst scenarios. We find an extremely luminous outburst of neutrinos with a peak luminosity of more than 4E54 erg/s for several milliseconds. This leads to an efficiency of about 1% for the annihilation of neutrinos with antineutrinos, corresponding to an average energy deposition rate of more than 1E52 erg/s and a total energy of about 1E50 erg deposited in electron-positron pairs around the collision site within 10ms. Although these numbers seem very favorable for gamma-ray burst scenarios, the pollution of the $e^\pm$ pair-plasma cloud with nearly 0.1$M_{\odot}$ of dynamically ejected baryons is 5 orders of magnitude too large. Therefore the formation of a relativistically expanding fireball that leads to a gamma-ray burst powered by neutrino emission from colliding neutron stars is definitely ruled out.

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arXiv1996

Numerical Models of Protoneutron Stars and Type-II Supernovae --- Recent Developments

The results of recent multi-dimensional simulations of type-II supernovae are reviewed. They show that convective instabilities in the collapsed stellar core might play an important role already during the first second after the formation of the supernova shock. Convectively unstable situations occur below and near the neutrinosphere as well as in the neutrino-heated region between the nascent neutron star and the supernova shock after the latter has stalled at a radius of typically 100-200 km. While convective overturn in the layer of neutrino energy deposition clearly helps the explosion to develop and potentially provides an explanation of strong mantle and envelope mixing, asphericities, and non-uniform 56Ni distribution observed in supernova SN 1987A, its presence and importance depends on the strength of the neutrino heating and thus on the size of the neutrino fluxes from the neutron star. Convection in the hot-bubble region can only develop if the growth timescale of the instabilities and the heating timescale are both shorter than the accretion timescale of the matter advected through the stagnant shock. For too small neutrino luminosities this requirement is not fulfilled and convective activity cannot develop, leading to very weak explosions or even fizzling models, just as in the one-dimensional situation. Convectively enhanced neutrino luminosities from the protoneutron star can therefore provide an essential condition for the explosion of the star. Very recent two-dimensional, self-consistent, general relativistic simulations of the cooling of a newly-formed neutron star demonstrate and confirm the possibility that Ledoux convection, driven by negative lepton number and

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arXiv1996

Ledoux-Convection in Protoneutron Stars --- a Clue to Supernova Nucleosynthesis?

Two-dimensional hydrodynamical simulations of the deleptonization of a newly formed neutron star were performed. Driven by negative lepton fraction and entropy gradients, convection starts near the neutrinosphere about 20-30 ms after core bounce, but moves deeper into the protoneutron star, and after about one second the whole protoneutron star is convective. The deleptonization of the star proceeds much faster than in the corresponding spherically symmetrical model because the lepton flux and the neutrino luminosities increase by up to a factor of two. The convection below the neutrinosphere raises the neutrinospheric temperatures and mean energies of the emitted neutrinos by 10-20%. This can have important implications for the supernova explosion mechanism and changes the detectable neutrino signal from the Kelvin-Helmholtz cooling of the protoneutron star. In particular, the enhanced electron neutrino flux relative to the electron antineutrino flux during the early post-bounce evolution might solve the overproduction problem of certain elements in the neutrino-heated ejecta in models of type-II supernova explosions.

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arXiv1995

Neutrinos from SN 1987A: Can they tell even more ?

The neutrino signal from SN~1987A provides an excellent opportunity to constrain physical theories for matter at extreme conditions and properties of particles that are produced in supernova (SN) cores. Phase transitions in the supranuclear equation of state (EOS) may change the cooling history of the collapsed stellar core and may even lead to a collapse of the protoneutron star to a black hole on a time scale of several seconds. The implications of such a scenario for the characteristics of the neutrino emission are tested. Moreover, the consequences of a reduction of the neutrino opacities by a possible suppression of the axial-vector interaction in a dense medium (Raffelt and Seckel 1994) are investigated. Since the strongest constraints from SN~1987A for the considered theories rely on the signal duration marked by the last three events in Kamiokande, we also discuss attempts to explain these events different from the standard protoneutron star cooling scenario. In particular, attention is focussed on the scenario of a rapid cooling of the protoneutron star within the time interval of the first 8 Kamiokande events or the IMB signal which is only about 2--$5\,{\rm s}$. Such a short neutrino emission could be the consequence of low neutrino opacities or convectively enhanced neutrino transport. Since the driving force of neutrino heating around the protoneutron star diminishes, the pressure support of the overlying material decreases and some matter might fall back to the neutron star. However, analytical estimates and numerical simulations indicate that the neutrino emission associated with the accretion of this material is hardly energetic enough to account for the last bunch of neutrinos at Kamiokande.

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arXiv1995

WHEN DO SUPERNOVA NEUTRINOS OF DIFFERENT FLAVORS HAVE SIMILAR LUMINOSITIES BUT DIFFERENT SPECTRA ?

Muon and tau neutrinos ($ν_x$) interact with protoneutron star matter only via neutral currents and exchange energy with the stellar gas predominantly by neutrino-electron scattering and neutrino-pair processes. In contrast, electron neutrinos and antineutrinos ($ν_e$ and $\barν_e$) are frequently absorbed and produced in charged-current mediated reactions with nucleons. Therefore the emergent $ν_e$ and $\barν_e$ originate from layers with lower temperatures further out in the star and are emitted with much lower characteristic spectral temperatures. In addition, a major contribution to the $ν_e$ and $\barν_e$ opacities is due to absorptions, while the opacity of $ν_x$ is strongly dominated by scattering reactions with nucleons and nuclei in which the $ν_x$ energy is (essentially) conserved. Therefore the $ν_x$ distribution is nearly isotropic when $ν_x$ decouple energetically and their outward diffusion is slowed down. In a generalized form to include this effect, the Stefan-Boltzmann Radiation Law can account for both the facts that $ν_e$ ($\barν_e$) and $ν_x$ emerge from the star with similar luminosities but with very different characteristic spectral temperatures. Simple analytical expressions to estimate the effect are given. If, as recently argued, even at densities significantly below nuclear matter density neutral-current scatterings were associated with considerable energy transfer between neutrino and target particle, one might expect spectral temperatures of $ν_x$ much closer to those of $ν_e$ and $\barν_e$. This is of relevance for the detection of neutrino signals from supernovae.

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arXiv1995

The First Second of a Type-II Supernova: Convection, Accretion, and Shock Propagation

One- and two-dimensional hydrodynamical simulations of neutrino-driven supernova explosions are discussed. The simulations cover the phase between the stagnation of the prompt shock and about one second after core bounce. Systematic variation of the neutrino fluxes from the neutrino sphere shows that the explosion energy, explosion time scale, initial mass of the protoneutron star, and explosive nucleosynthesis of iron-group elements depend sensitively on the strength of the neutrino heating during the first few 100 ms after shock formation. Convective overturn in the neutrino-heated region behind the shock is a crucial help for the explosion only in a narrow window of neutrino luminosities. Here powerful explosions can be obtained only in the multi-dimensional case. For higher core-neutrino fluxes also spherically symmetrical models yield energetic explosions, while for lower luminosities even with convection no strong explosions occur.

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arXiv1994

Reduced Neutrino Opacities and the SN 1987A Signal

Axial-vector interactions of neutrinos and axions with a medium are based on structure functions which cannot be calculated in simple perturbative terms for nuclear densities. We use the SN~1987A neutrino signal duration to estimate the range of allowed neutrino opacities of a supernova (SN) core. We perform numerical simulations of the evolution from the hot, lepton-rich post-collapse stage to the cold, deleptonized neutron star and compare the predicted neutrino signal characteristics with the SN~1987A data. Besides the neutrino opacities we vary the mass, temperature profile, and equation of state of the initial model. Within plausible limits of these quantities the neutrino opacity cannot be much smaller than its ``standard'' value. This limits the width of the spin-density structure function to much smaller values than indicated by a perturbative calculation, implying that bremsstrahlung processes such as axion emission probably saturate at around 10\% nuclear density. A substantial suppression at yet larger densities that might have been expected does not seem to occur.

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