Core Bounce Research Articles

Gravitational waves from supernova matter

We have performed a set of 11 three-dimensional magnetohydrodynamical (MHD) core-collapse supernova simulations in order to investigate the dependences of the gravitational wave signal on the progenitor's initial conditions. We study the effects of the initial central angular velocity and different variants of neutrino transport. Our models are started up from a 15M⊙ progenitor and incorporate an effective general relativistic gravitational potential and a finite temperature nuclear equation of state. Furthermore, the electron flavour neutrino transport is tracked by efficient algorithms for the radiative transfer of massless fermions. We find that non- and slowly rotating models show gravitational wave emission due to prompt- and lepton driven convection that reveals details about the hydrodynamical state of the fluid inside the protoneutron stars. Furthermore we show that protoneutron stars can become dynamically unstable to rotational instabilities at T/|W| values as low as ∼2% at core bounce. We point out that the inclusion of deleptonization during the postbounce phase is very important for the quantitative gravitational wave (GW) prediction, as it enhances the absolute values of the gravitational wave trains up to a factor of ten with respect to a lepton-conserving treatment.

The influence of model parameters on the prediction of gravitational wave signals from stellar core collapse

We present the gravitational wave (GW) analysis of an extensive series of 3D MHD core-collapse simulations. Our 25 models are launched from a 15 solar mass progenitor, a spherically symmetric effective general relativistic potential, the Lattimer-Swesty or the Shen equation of state (EoS), and a neutrino parametrisation scheme which is accurate until about 5ms postbounce. For 3 representative models, we also include long-term neutrino physics by means of a leakage scheme. Non- or only slowly rotating models show GW emission due to prompt and proto-neutron star convection, allowing the distinction between the two different nuclear EoS. For moderately or fast rotation rates models, we find, in agreement with recent results, only a type I GW signature at core bounce. Models which are set up with an initial central angular velocity of >~ 2pi rad/s emit GWs due to the low T/|W| dynamical instability during the postbounce phase. Weak B-fields do not notably influence the dynamical evolution of the core and thus the GW emission. However, for strong initial poloidal B-fields (~1e12 G),flux-freezing and field winding leads to conditions where P_{mag}/P_{mat} ~ 1, causing the onset of a jet-like supernova explosion and hence the emission of a type IV GW signal. In contradiction to axisymmetric simulations, we find evidence that nonaxisymmetric fluid modes can counteract or even suppress jet formation for models with strong initial toroidal B-fields. We point out that the inclusion of the deleptonisation during the postbounce phase is an indispensable issue for the quantitative prediction of GWs from core-collapse supernovae, as it can alter the GW amplitude up to a factor of 10 compared to a pure hydrodynamical treatment.

Emergence of hyperons in failed supernovae with short neutrino bursts

We present that the appearance of hyperons plays a crucial role in failed supernovae from massive stars of ∼ 40 M ⊙ . The quick dynamics from the gravitational collapse to the black hole leads to extreme conditions, reaching high densities and temperatures to have strangeness particles, within 1 s after the core bounce. The associated neutrino bursts are short and energetic, being different from ordinary supernova neutrinos, and may provide the information on the strangeness in dense matter. The end point of the duration of neutrino burst is determined by the stiffness of EOS and the appearance of new particle is the trigger of the termination due to the black hole formation. By measuring the duration of burst and the energy spectrum, one can constrain the appearance of exotics. The event numbers of neutrinos from the black-hole-forming collapse at the terrestrial detector are found large enough to utilize this phenomena as a target of neutrino astronomy and a probe of dense matter with strangeness.

Neutrino oscillations in MHD supernova explosions

We calculate the neutrino oscillations numerically in magnetohydrodynamic (MHD) explosion models to see how asphericity has impacts on neutrino spectra. Magneto-driven explosions are one of the most attracting scenarios for producing large scale departures from spherical symmetric geometry, that are reported by many observational data. We find that the event rates at Super-Kamiokande (SK) seen from the polar direction (e.g., the rotational axis of the supernovae) decrease when the shock wave is propagating through H-resonance. In addition, we find that L-resonance in this situation becomes non-adiabatic, and the effect of L-resonance appears in the neutrino signal, because the MHD shock can propagate to the stellar surface without shock-stall after core bounce, and the shock reaches the L-resonance at earlier stage than the conventional spherical supernova explosion models. Our results suggest that we may obtain the observational signatures of the two resonances in SK for Galactic supernova.

SHOCK BREAKOUT FROM TYPE Ia SUPERNOVA

The mode of explosive burning in Type Ia SNe remains an outstanding problem. It is generally thought to begin as a subsonic deflagration, but this may transition into a supersonic detonation (the DDT). We argue that this transition leads to a breakout shock, which would provide the first unambiguous evidence that DDTs occur. Its main features are a hard X-ray flash (~20 keV) lasting ~0.01 s with a total radiated energy of ~10^{40} ergs, followed by a cooling tail. This creates a distinct feature in the visual light curve, which is separate from the nickel decay. This cooling tail has a maximum absolute visual magnitude of M_V = -9 to -10 at approximately 1 day, which depends most sensitively on the white dwarf radius at the time of the DDT. As the thermal diffusion wave moves in, the composition of these surface layers may be imprinted as spectral features, which would help to discern between SN Ia progenitor models. Since this feature should accompany every SNe Ia, future deep surveys (e.g., m=24) will see it out to a distance of approximately 80 Mpc, giving a maximum rate of ~60/yr. Archival data sets can also be used to study the early rise dictated by the shock heating (at about 20 days before maximum B-band light). A similar and slightly brighter event may also accompany core bounce during the accretion induced collapse to a neutron star, but with a lower occurrence rate.

Bayesian reconstruction of gravitational wave burst signals from simulations of rotating stellar core collapse and bounce

Presented in this paper is a technique that we propose for extracting the physical parameters of a rotating stellar core collapse from the observation of the associated gravitational wave signal from the collapse and core bounce. Data from interferometric gravitational wave detectors can be used to provide information on the mass of the progenitor model, precollapse rotation, and the nuclear equation of state. We use waveform libraries provided by the latest numerical simulations of rotating stellar core collapse models in general relativity, and from them create an orthogonal set of eigenvectors using principal component analysis. Bayesian inference techniques are then used to reconstruct the associated gravitational wave signal that is assumed to be detected by an interferometric detector. Posterior probability distribution functions are derived for the amplitudes of the principal component analysis eigenvectors, and the pulse arrival time. We show how the reconstructed signal and the principal component analysis eigenvector amplitude estimates may provide information on the physical parameters associated with the core collapse event.

Perturbation analysis of a general polytropic homologously collapsing stellar core

For dynamic background models of Goldreich & Weber and Lou & Cao, we examine three-dimensional perturbation properties of oscillations and instabilities in a general polytropic homologously collapsing stellar core of a relativistically hot medium with a polytropic index γ= 4/3. Perturbation behaviours, especially internal gravity g modes, depend on the variation of specific entropy in the collapsing core. Among possible perturbations, we identify acoustic p modes and surface f modes as well as internal gravity g+ and g− modes. As in stellar oscillations of a static star, we define g+ and g− modes by the sign of the Brunt–Väisälä buoyancy frequency squared for a collapsing stellar core. A new criterion for the onset of instabilities is established for a homologous stellar core collapse. We demonstrate that the global energy criterion of Chandrasekhar is insufficient to warrant the stability of general polytropic equilibria. We confirm the acoustic p-mode stability of Goldreich & Weber, even though their p-mode eigenvalues appear in systematic errors. Unstable modes include g− modes and sufficiently high-order g+ modes, corresponding to core instabilities. Such instabilities occur before the stellar core bounce, in contrast to instabilities in other models of supernova (SN) explosions. The breakdown of spherical symmetry happens earlier than expected in numerical simulations so far. The formation and motion of the central compact object are speculated to be much affected by such g-mode instabilities. By estimates of typical parameters, unstable low-order l= 1 g-modes may produce initial kicks of the central compact object. Other high-order and high-degree unstable g modes may shred the nascent neutron core into pieces without an eventual compact remnant (e.g. SN 1987A). Formation of binary pulsars and planets around neutron stars might originate from unstable l= 2 g-modes and high-order high-degree g modes, respectively.

Non-spherical core-collapse supernovae: evolution towards homologous expansion**This paper is published as part of a collection in honour of Todd Dupont's 65th birthday.

We study the hydrodynamic evolution of a non-spherical core-collapse supernova in multidimensions. We begin our study from the moment of shock revival and continue for the first week after the explosion when the expansion of the supernova ejecta becomes homologous. We observe the growth and interaction of Richtmyer–Meshkov, Rayleigh–Taylor and Kelvin–Helmholtz instabilities resulting in an extensive mixing of the heavy elements throughout the ejecta.We obtain a series of models at progressively higher resolution and provide a preliminary discussion of numerical convergence. Unlike in previous studies, our computations are performed in a single domain. Periodic mesh mapping (Couch et al 2009 Astrophys. J. 696 953–70 and Kifonidis et al 2006 Astron. Astrophys. 453 661–78) is avoided. This is made possible by employing an adaptive mesh refinement strategy in which the computational workload (defined as the product of the total number of computational cells and the length of the time step) is monitored and, if necessary, limited.Our results are in overall good agreement with the simulations reported by Kifonidis et al (2006 Astron. Astrophys. 453 661–78). We demonstrate, however, that the amount of mixing and the kinematic properties of radioactive species (i.e. 56Ni) are extremely anisotropic. In particular, we find that the model displays a strong tendency to expand laterally away from the equatorial plane towards the poles. Although this tendency is usually attributed to numerical artefacts characteristic of computations with assumed symmetry (axis effect), the observed behaviour can be largely explained by the structure of the neutrino-driven explosion model. Future studies are needed to establish to what degree the morphology of a young supernova remnant can be used to probe early stages of the explosion especially in situations when the standing accretion shock instability is at work within the first second after core bounce.

NUCLEOSYNTHESIS IN ELECTRON CAPTURE SUPERNOVAE OF ASYMPTOTIC GIANT BRANCH STARS

We examine nucleosynthesis in the electron capture supernovae of progenitor AGB stars with an O-Ne-Mg core (with the initial stellar mass of 8.8 M_\odot). Thermodynamic trajectories for the first 810 ms after core bounce are taken from a recent state-of-the-art hydrodynamic simulation. The presented nucleosynthesis results are characterized by a number of distinct features that are not shared with those of other supernovae from the collapse of stars with iron core (with initial stellar masses of more than 10 M_\odot). First is the small amount of 56Ni (= 0.002-0.004 M_\odot) in the ejecta, which can be an explanation for observed properties of faint supernovae such as SNe 2008S and 1997D. In addition, the large Ni/Fe ratio is in reasonable agreement with the spectroscopic result of the Crab nebula (the relic of SN 1054). Second is the large production of 64Zn, 70Ge, light p-nuclei (74Se, 78Kr, 84Sr, and 92Mo), and in particular, 90Zr, which originates from the low Y_e (= 0.46-0.49, the number of electrons per nucleon) ejecta. We find, however, that only a 1-2% increase of the minimum Y_e moderates the overproduction of 90Zr. In contrast, the production of 64Zn is fairly robust against a small variation of Y_e. This provides the upper limit of the occurrence of this type of events to be about 30% of all core-collapse supernovae.

NEUTRINO MASS SPECTRUM FROM NEUTRINO SPIN-FLIP-DRIVEN GRAVITATIONAL WAVES

Neutrino (ν) oscillations during the core collapse and bounce of a supernova (SN) are shown to generate the most powerful detectable gravitational wave (GW) bursts. The SN neutronization phase produces mainly electron (νe) neutrinos, the oscillations of which must take place within a few mean-free paths of their resonance surface located near their neutrinosphere. Here we characterize the GW signals produced by spin-flip oscillations inside the fast-rotating protoneutron star in the SN core. In this novel mechanism, the release of both the oscillation-produced νμ's, ντ's and the spin-flip-driven GW pulse provides a unique emission offset [Formula: see text] for measuring the ν travel time to Earth. As massive ν's get noticeably delayed on its journey to Earth with respect to the GW, they generate over the oscillation transient, the accurate measurement of this time-of-flight delay by SNEWS + LIGO, VIRGO, BBO, DECIGO, etc. can assess the absolute ν mass spectrum straightforwardly.

Equation-of-state dependent features in shock-oscillation modulated neutrino and gravitational-wave signals from supernovae

We present two-dimensional (axisymmetric) neutrino-hydrodynamic simulations of the long-time accretion phase of a 15 Mprogen- itor star after core bounce and before the launch of a supernova explosion, when non-radial hydrodynamic instabilities like convection occur in different regions of the collapsing stellar core and the standing accretion shock instability (SASI) leads to large-amplitude os- cillations of the stalled shock with a period of tens of milliseconds. Our simulations were performed with the Prometheus-Vertex code, which includes a multi-flavor, energy-dependent neutrino transport scheme and employs an effective relativistic gravitational potential. Testing the influence of a stiff and a soft equation of state for hot neutron star matter, we find that the non-radial mass motions in the supernova core impose a time variability on the neutrino and gravitational-wave signals with larger amplitudes, as well as higher frequencies in the case of a more compact nascent neutron star. After the prompt shock-breakout burst of electron neutrinos, a more compact accreting remnant produces higher neutrino luminosities and higher mean neutrino energies. The observ- able neutrino emission in the SASI sloshing direction exhibits a modulation of several ten percent in the luminosities and around 1 MeV in the mean energies with most power at typical SASI frequencies between roughly 20 and 100 Hz. The modulation is caused by quasi-periodic variations in the mass accretion rate of the neutron star in each hemisphere. At times later than ∼50-100 ms after bounce, the gravitational-wave amplitude is dominated by the growing low-frequency (<200 Hz) signal associated with anisotropic neutrino emission. A high-frequency wave signal results from nonradial gas flows in the outer layers of the anisotropically accreting neutron star. Right after bounce such nonradial mass motions occur due to prompt post-shock convection in both considered cases and contribute mostly to the early wave production around 100 Hz. Later they are instigated by the SASI and by convective overturn that vigorously stir the neutrino-heating and cooling layers, and also by convective activity developing below the neutrinosphere. The gravitational-wave power then peaks at about 300-800 Hz, connected to changes in the mass quadrupole moment on a timescale of milliseconds. Distinctively higher spectral frequencies originate from the more compact and more rapidly contracting neutron star. Both the neutrino and gravitational-wave emission therefore carry information that is characteristic of the properties of the nuclear equation of state in the hot remnant. The detectability of the SASI effects in the neutrino and gravitational-wave signals is briefly discussed.

EMERGENCE OF HYPERONS IN FAILED SUPERNOVAE: TRIGGER OF THE BLACK HOLE FORMATION

We investigate the emergence of strange baryons in the dynamical collapse of a nonrotating massive star to a black hole using neutrino-radiation hydrodynamical simulations in general relativity. By following the dynamical formation and collapse of a nascent proto-neutron star from the gravitational collapse of a 40 M☉ star adopting a new hyperonic equation-of-state (EOS) table, we show that the hyperons do not appear at the core bounce but populate quickly ∼0.5–0.7 s after the bounce to trigger the recollapse to a black hole. They start to show up off center owing to high temperatures and later prevail at center when the central density becomes high enough. The neutrino emission from the accreting proto-neutron star with the hyperonic EOS stops much earlier than the corresponding case with a nucleonic EOS, while the average energies and luminosities are quite similar in the two cases. These features of the neutrino signal are a potential probe of the emergence of new degrees of freedom inside the black hole forming collapse.

Dynamics and Neutrino Signal of Black Hole Formation in Nonrotating Failed Supernovae. II. Progenitor Dependence

We study the progenitor dependence of black hole formation and its associated neutrino signal due to the gravitational collapse of a nonrotating massive star, following on our previous study of a single progenitor model. We aim to clarify whether the dynamical evolution toward black hole formation occurs in the same manner for different progenitors and to examine whether the characteristic of neutrino bursts having short duration and rapidly increasing average energies is a general one. We perform numerical simulations using general relativistic neutrino radiation hydrodynamics to follow the dynamical evolution from the collapse of 40 and 50 M☉ presupernova models to black hole formation via contracting proto-neutron stars. For the three progenitor models studied in this paper, we find that black hole formation occurs ~0.4-1.5 s after core bounce through an increase of the proto-neutron star mass, accompanied by a short and energetic neutrino burst. The density profile of the progenitor is important in determining the accretion rate onto the proto-neutron star and, therefore, the duration of the neutrino burst. We compare the neutrino bursts of black hole-forming events from different progenitors and discuss whether they can be used to probe clearly the progenitor and the dense matter.

Role of dense matter in collective supernova neutrino transformations

For neutrinos streaming from a supernova core, dense matter suppresses self-induced flavor transformations if the electron density n{sub e} significantly exceeds the neutrino density n{sub {nu}} in the conversion region. If n{sub e} is comparable to n{sub {nu}}, one finds multiangle decoherence, whereas the standard self-induced transformation behavior requires that in the transformation region n{sub {nu}} is safely above n{sub e}. This condition need not be satisfied in the early phase after the supernova core bounce. Our new multiangle effect is a subtle consequence of neutrinos traveling on different trajectories when streaming from a source that is not pointlike.

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 possibility 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.

Gravitational waves from 3D MHD core collapse simulations

We present the gravitational wave analyses from rotating (model s15g) and nearly non-rotating (model s15h) 3D MHD core collapse supernova simulations at bounce and during the first couple of ten milliseconds afterwards. The simulations are launched from 15 M� progenitor models stemming from stellar-evolution calculations. Gravity is implemented by a spherically symmetric effective general relativistic potential. The input physics uses the Lattimer-Swesty equation of state for hot, dense matter and a neutrino parametrisation scheme that is accurate until the first few ms after bounce. The 3D simulations allow us to study features already known from 2D simulations, as well as nonaxisymmetric effects. In agreement with recent results, we find only type I gravitational wave signals at core bounce. In the later stage of the simulations, one of our models (s15g) shows nonaxisymmetric gravitational wave emission caused by a low T/|W| dynamical instability, while the other model radiates gravitational waves due to a convective instability in the protoneutron star. The total energy released in gravitational waves within the considered time intervals is 1.52 × 10 −7 M� (s15g) and 4.72 × 10 −10 M� (s15h). Both core collapse simulations indicate that corresponding events in our Galaxy would be detectable either by the LIGO or Advanced LIGO detector.

Relativistic jets and long-duration gamma-ray bursts from the birth of magnetars

Abstract We present time-dependent axisymmetric magnetohydrodynamic simulations of the interaction of a relativistic magnetized wind produced by a proto-magnetar with a surrounding stellar envelope, in the first ∼10 s after core collapse. We inject a super-magnetosonic wind with into a cavity created by an outgoing supernova shock. A strong toroidal magnetic field builds up in the bubble of plasma and magnetic field that is at first inertially confined by the progenitor star. This drives a jet out along the polar axis of the star, even though the star and the magnetar wind are each spherically symmetric. The jet has the properties needed to produce a long-duration gamma-ray burst (GRB). At ∼5 s after core bounce, the jet has escaped the host star and the Lorentz factor of the material in the jet at large radii ∼1011 cm is similar to that in the magnetar wind near the source. Most of the spindown power of the central magnetar escapes via the relativistic jet. There are fluctuations in the Lorentz factor and energy flux in the jet on a ∼ 0.01–0.1 s time-scale. These may contribute to variability in GRB emission (e.g. via internal shocks).

Gravitational waves from core-collapse supernovae and their related compact objects

Core-collapse supernovae have been supposed to be one of the most plausible sources of gravitational waves. Based on a series of our magnetohydrodynamic core-collapse simulations, we find that the gravitational amplitudes at core bounce can be within the detection limits for the currently running laser-interferometers for a galactic supernova if the central core rotates sufficiently rapidly. This is regardless of the difference of the realistic equations of state and the possible occurrence of the QCD phase transition near core bounce. Even if the core rotates slowly, we point out that the gravitational waves generated from anisotropic neutrino radiation in the postbounce phase due to the standing accretion shock instability (SASI) could be within the detection limits of the detectors in the next generation such as LCGT and the advanced LIGO for the galactic source. Since the waveforms significantly depend on the exploding scenarios, our results suggest that we can obtain the information about the long-veiled explosion mechanism from the gravitational wave signals. Furthermore we discuss the gravitational wave background (GWB) from the explosions of Pop III stars and show that the GWB from Pop III, depending on their formation rates, can be large enough to be within the detection limits of future planned interferometers such as DECIGO and BBO in the frequency interval of ~0.1-1 Hz. This means that the detections of GW background from Pop III stars can be an important tool to supply the information about the star formation history in the early universe.

Dynamics and Neutrino Signal of Black Hole Formation in Nonrotating Failed Supernovae. I. Equation of State Dependence

We study the black hole formation and the neutrino signal from the gravitational collapse of a non-rotating massive star of 40 Msun. Adopting two different sets of realistic equation of state (EOS) of dense matter, we perform the numerical simulations of general relativistic neutrino-radiation hydrodynamics under the spherical symmetry. We make comparisons of the core bounce, the shock propagation, the evolution of nascent proto-neutron star and the resulting re-collapse to black hole to reveal the influence of EOS. We also explore the influence of EOS on the neutrino emission during the evolution toward the black hole formation. We find that the speed of contraction of the nascent proto-neutron star, whose mass increases fast due to the intense accretion, is different depending on the EOS and the resulting profiles of density and temperature differ significantly. The black hole formation occurs at 0.6-1.3 sec after bounce when the proto-neutron star exceeds its maximum mass, which is crucially determined by the EOS. We find that the average energies of neutrinos increase after bounce because of rapid temperature increase, but at different speeds depending on the EOS. The duration of neutrino emission up to the black hole formation is found different according to the different timing of re-collapse. These characteristics of neutrino signatures are distinguishable from those for ordinary proto-neutron stars in successful core-collapse supernovae. We discuss that a future detection of neutrinos from black-hole-forming collapse will contribute to reveal the black hole formation and to constrain the EOS at high density and temperature.

On the Conditions for Neutron‐rich Gamma‐Ray Burst Outflows

We calculate the structure and neutron content of neutrino-heated magnetohydrodynamic winds driven from the surface of newly formed magnetars (protomagnetars) and from the midplane of hyperaccreting disks, two of the possible central engines for GRBs and hyperenergetic SNe. Both the surface of protomagnetars and the midplane of neutrino-cooled accretion flows (NDAFs) are electron degenerate and neutron-rich (neutron-to-proton ratio -->n/p 1). If this substantial free neutron excess is preserved to large radii in ultrarelativistic outflows, several important observational consequences may result. Weak interaction processes, however, can drive -->n/p to ~1 in the nondegenerate regions that obtain just above the surfaces of NDAFs and protomagnetars. Our calculations show that mildly relativistic (Lorentz factor -->? 10) neutron-rich outflows from NDAFs are possible in the presence of a strong poloidal magnetic field. However, neutron-rich winds possess a minimum mass-loss rate that likely precludes simultaneously neutron-rich and ultrarelativistic ( -->? 100) NDAF winds accompanying a substantial accretion power. In contrast, protomagnetars are capable of producing neutron-rich long-duration GRB outflows ~10-30 s following core bounce for submillisecond rotation periods; such outflows would, however, accompany only extremely energetic events, in which the GRB+SN energy budget exceeds ~ -->4 ? 1052 ergs. Neutron-rich highly relativistic outflows may also be produced during some short-duration GRBs by geometrically thick accretion disks formed from compact object mergers. The implications for r-process nucleosynthesis, optical transients due to nonrelativistic neutron-rich winds, and nickel production in protomagnetar and NDAF winds are also briefly discussed.