Neutrino-driven Core-collapse Supernova Research Articles

Nucleosynthesis in outflows of compact objects and detection prospects of associated kilonovae

ABSTRACT We perform a comparative analysis of nucleosynthesis yields from binary neutron star (BNS) mergers, black hole-neutron star (BHNS) mergers, and core-collapse supernovae (CCSNe) with the goal of determining which are the most dominant sources of r-process enrichment observed in stars. We find that BNS and BHNS binaries may eject similar mass distributions of robust r-process nuclei post-merger (up to third peak and actinides, A ∼ 200−240), after accounting for the volumetric event rates. Magnetorotational (MR) CCSNe likely undergo a weak r-process (up to A ∼ 140) and contribute to the production of light element primary process (LEPP) nuclei, whereas typical thermal, neutrino-driven CCSNe only synthesize up to first r-process peak nuclei (A ∼ 80−90). We also find that the upper limit to the rate of MR CCSNe is $\lesssim 1~{{\ \rm per\ cent}}$ the rate of typical thermal CCSNe; if the rate was higher, then weak r-process nuclei would be overproduced. Although the largest uncertainty is from the volumetric event rate, the prospects are encouraging for confirming these rates in the next few years with upcoming surveys. Using a simple model to estimate the resulting kilonova light curve from mergers and our set of fiducial merger parameters, we predict that ∼7 BNS and ∼2 BHNS events will be detectable per year by the Vera C. Rubin Observatory (LSST), with prior gravitational wave (GW) triggers.

Inferring Type II-P Supernova Progenitor Masses from Plateau Luminosities

Connecting observations of core-collapse supernova explosions to the properties of their massive star progenitors is a long-sought, and challenging, goal of supernova science. Recently, Barker et al. presented bolometric light curves for a landscape of progenitors from spherically symmetric neutrino-driven core-collapse supernova (CCSN) simulations using an effective model. They find a tight relationship between the plateau luminosity of the Type II-P CCSN light curve and the terminal iron-core mass of the progenitor. Remarkably, this allows us to constrain progenitor properties with photometry alone. We analyze a large observational sample of Type II-P CCSN light curves and estimate a distribution of iron-core masses using the relationship of Barker et al. The inferred distribution matches extremely well with the distribution of iron-core masses from stellar evolutionary models and namely, contains high-mass iron cores that suggest contributions from very massive progenitors in the observational data. We use this distribution of iron-core masses to infer minimum and maximum masses of progenitors in the observational data. Using Bayesian inference methods to locate optimal initial mass function parameters, we find and solar masses for the observational data.

2D numerical study for magnetic field dependence of neutrino-driven core-collapse supernova models

ABSTRACT We study the effects of the magnetic field on the dynamics of non-rotating stellar cores by performing 2D, magnetohydrodynamic (MHD) simulations. To this end, we have updated our neutrino-radiation-hydrodynamics supernova code to include MHD employing a divergence cleaning method with both careful treatments of finite volume and area reconstructions. By changing the initial strength of the magnetic field, the evolution of 15.0, 18.4, and $27.0\,\rm M_\odot$ pre-supernova progenitors is investigated. An intriguing finding in our study is that the neutrino-driven explosion occurs regardless of the strength of the initial magnetic field. For the 2D models presented in this work, the neutrino heating is the main driver for the explosion, whereas the magnetic field secondary contributes to the pre-explosion dynamics. Our results show that the strong magnetic field weakens the growth of the neutrino-driven turbulence in the small scale compared to the weak magnetic field. This results in the slower increase of the turbulent kinetic energy in the post-shock region, leading to the slightly delayed onset of the shock revival for models with the stronger initial magnetic field.

On the Development of Multidimensional Progenitor Models for Core-collapse Supernovae

Abstract Multidimensional hydrodynamic simulations of shell convection in massive stars suggest the development of aspherical perturbations that may be amplified during iron core collapse. These perturbations have a crucial and qualitative impact on the delayed neutrino-driven core-collapse supernova explosion mechanism by increasing the total stress behind the stalled shock. In this paper, we investigate the properties of a 15 model evolved in one, two, and three dimensions (3D) for the final ∼424 s before gravitational instability and iron core collapse using Modules for Experiments in Stellar Astrophysics (MESA) and the FLASH simulation framework. We find that just before collapse, our initially perturbed fully 3D model reaches angle-averaged convective velocity magnitudes of ≈240–260 km s−1 in the Si- and O-shell regions with a Mach number of ≈0.06. We find the bulk of the power in the O-shell resides at large scales, characterized by spherical harmonic orders (ℓ) of 2–4, while the Si-shell shows broad spectra on smaller scales of . Both convective regions show an increase in power at near collapse. We show that the 1D MESA model agrees with the convective velocity profile and speeds of the Si-shell when compared to our highest resolution 3D model. However, in the O-shell region, we find that MESA predicts speeds approximately four times slower than all of our 3D models suggest. All eight of the multidimensional stellar models considered in this work are publicly available.

The impact of asymmetric neutrino emissions on nucleosynthesis in core-collapse supernovae

ABSTRACT We investigate the impact of asymmetric neutrino emissions on the explosive nucleosynthesis in neutrino-driven core-collapse supernovae (CCSNe). We find that the asymmetric emissions tend to yield larger amounts of proton-rich ejecta (electron fraction, Ye > 0.51) in the hemisphere of the higher νe emissions, meanwhile neutron-rich matter (Ye < 0.49) are ejected in the opposite hemisphere of the higher ${\bar{\nu }}_{\rm e}$ emissions. For larger asymmetric cases with $\ge 30\, {\rm per\, cent}$, the neutron-rich ejecta is abundantly produced, in which there are too much elements heavier than Zn compared to the solar abundances. This may place an upper limit of the asymmetric neutrino emissions in CCSNe. The characteristic features are also observed in elemental distribution; (1) abundances lighter than Ca are insensitive to the asymmetric neutrino emissions; (2) the production of Zn and Ge is larger in the neutron-rich ejecta even for smaller asymmetric cases with $\le 10\, {\rm per\, cent}$. We discuss these observational consequences, which may account for the (anti)correlations among asymmetries of heavy elements and neutron star kicks in supernova remnants (SNRs). Future SNR observations of the direct measurement for the mass and spatial distributions of α elements, Fe, Zn, and Ge will provide us the information on the asymmetric degree of neutrino emissions.

Three-dimensional simulations of neutrino-driven core-collapse supernovae from low-mass single and binary star progenitors

We present a suite of seven 3D supernova simulations of non-rotating low-mass progenitors using multi-group neutrino transport. Our simulations cover single star progenitors with zero-age main sequence masses between $9.6 M_\odot$ and $12.5 M_\odot$ and (ultra)stripped-envelope progenitors with initial helium core masses between $2.8 M_\odot$ and $3.5 M_\odot$. We find explosion energies between $0.1\,\mathrm{Bethe}$ and $0.4\,\mathrm{Bethe}$, which are still rising by the end of the simulations. Although less energetic than typical events, our models are compatible with observations of less energetic explosions of low-mass progenitors. In six of our models, the mass outflow rate already exceeds the accretion rate onto the proto-neutron star, and the mass and angular momentum of the compact remnant have closely approached their final value, barring the possibility of later fallback. While the proto-neutron star is still accelerated by the gravitational tug of the asymmetric ejecta, the acceleration can be extrapolated to obtain estimates for the final kick velocity. We obtain gravitational neutron star masses between $1.22 M_\odot$ and $1.44 M_\odot$, kick velocities between $11\, \mathrm{km}\, \mathrm{s}^{-1}$ and $695\, \mathrm{km}\, \mathrm{s}^{-1}$, and spin periods from $20\, \mathrm{ms}$ to $2.7\,\mathrm{s}$, which suggests that typical neutron star birth properties can be naturally obtained in the neutrino-driven paradigm. We find a loose correlation between the explosion energy and the kick velocity. There is no indication of spin-kick alignment, but a correlation between the kick velocity and the neutron star angular momentum, which needs to be investigated further as a potential point of tension between models and observations.

The impact of vorticity waves on the shock dynamics in core-collapse supernovae

Convective perturbations arising from nuclear shell burning can play an important role in propelling neutrino-driven core-collapse supernova explosions. In this work, we analyze the impact of vorticity waves on the shock dynamics and the post-shock flow using the solution of the linear hydrodynamics equations. We show that the entropy perturbations generated by the interaction of the shock with vorticity waves may play a dominant role in generating buoyancy-driven turbulence in the gain region. We estimate that the resulting reduction in the critical luminosity is 17-24%, which approximately agrees with the results of three-dimensional neutrino-hydrodynamics simulations.

The Status of Multi-Dimensional Core-Collapse Supernova Models

AbstractModels of neutrino-driven core-collapse supernova explosions have matured considerably in recent years. Explosions of low-mass progenitors can routinely be simulated in 1D, 2D, and 3D. Nucleosynthesis calculations indicate that these supernovae could be contributors of some lighter neutron-rich elements beyond iron. The explosion mechanism of more massive stars remains under investigation, although first 3D models of neutrino-driven explosions employing multi-group neutrino transport have become available. Together with earlier 2D models and more simplified 3D simulations, these have elucidated the interplay between neutrino heating and hydrodynamic instabilities in the post-shock region that is essential for shock revival. However, some physical ingredients may still need to be added/improved before simulations can robustly explain supernova explosions over a wide range of progenitors. Solutions recently suggested in the literature include uncertainties in the neutrino rates, rotation, and seed perturbations from convective shell burning. We review the implications of 3D simulations of shell burning in supernova progenitors for the ‘perturbations-aided neutrino-driven mechanism,’ whose efficacy is illustrated by the first successful multi-group neutrino hydrodynamics simulation of an 18 solar mass progenitor with 3D initial conditions. We conclude with speculations about the impact of 3D effects on the structure of massive stars through convective boundary mixing.

Systematic features of axisymmetric neutrino-driven core-collapse supernova models in multiple progenitors

Abstract We present an overview of two-dimensional (2D) core-collapse supernova simulations employing a neutrino transport scheme by the isotropic diffusion source approximation. We study 101 solar-metallicity, 247 ultra metal-poor, and 30 zero-metal progenitors covering zero-age main sequence mass from 10.8 M⊙ to 75.0 M⊙. Using the 378 progenitors in total, we systematically investigate how the differences in the structures of these multiple progenitors impact the hydrodynamics evolution. By following a long-term evolution over 1.0 s after bounce, most of the computed models exhibit neutrino-driven revival of the stalled bounce shock at ∼200–800 ms postbounce, leading to the possibility of explosion. Pushing the boundaries of expectations in previous one-dimensional studies, our results confirm that the compactness parameter ξ that characterizes the structure of the progenitors is also a key in 2D to diagnosing the properties of neutrino-driven explosions. Models with high ξ undergo high ram pressure from the accreting matter onto the stalled shock, which affects the subsequent evolution of the shock expansion and the mass of the protoneutron star under the influence of neutrino-driven convection and the standing accretion-shock instability. We show that the accretion luminosity becomes higher for models with high ξ, which makes the growth rate of the diagnostic explosion energy higher and the synthesized nickel mass bigger. We find that these explosion characteristics tend to show a monotonic increase as a function of the compactness parameter ξ.

Three-dimensional simulations of SASI- and convection-dominated core-collapse supernovae

We investigate the effect of dimensionality on the transition to explosion in neutrino-driven core-collapse supernovae. Using parameterized hydrodynamic simulations of the stalled supernova shock in one-, two- (2D), and three spatial dimensions (3D), we systematically probe the extent to which hydrodynamic instabilities alone can tip the balance in favor of explosion. In particular, we focus on systems that are well into the regimes where the Standing Accretion Shock Instability (SASI) or neutrino-driven convection dominate the dynamics, and characterize the difference between them. We find that SASI-dominated models can explode with up to ~20% lower neutrino luminosity in 3D than in 2D, with the magnitude of this difference decreasing with increasing resolution. This improvement in explosion conditions is related to the ability of spiral modes to generate more non-radial kinetic energy than a single sloshing mode, increasing the size of the average shock radius, and hence generating better conditions for the formation of large-scale, high-entropy bubbles. In contrast, convection-dominated explosions show a smaller difference in their critical heating rate between 2D and 3D (<8%), in agreement with previous studies. The ability of our numerical implementation to maintain arbitrary symmetries is quantified with a set of SASI-based tests. We discuss implications for the diversity of explosion paths in a realistic supernova environment.

EFFECTS OF ROTATION ON STOCHASTICITY OF GRAVITATIONAL WAVES IN THE NONLINEAR PHASE OF CORE-COLLAPSE SUPERNOVAE

By performing three-dimensional (3D) simulations that demonstrate the neutrino-driven core-collapse supernovae aided by the standing accretion shock instability (SASI), we study how the spiral modes of the SASI can have impacts on the properties of the gravitational-wave (GW) emission. To see the effects of rotation in the non-linear postbounce phase, we give a uniform rotation on the flow advecting from the outer boundary of the iron core, whose specific angular momentum is assumed to agree with recent stellar evolution models. We compute fifteen 3D models in which the initial angular momentum as well as the input neutrino luminosities from the protoneutron star are changed in a systematic manner. By performing a ray-tracing analysis, we accurately estimate the GW amplitudes generated by anisotropic neutrino emission. Our results show that the gravitational waveforms from neutrinos in models that include rotation exhibit a common feature otherwise they vary much more stochastically in the absence of rotation. The breaking of the stochasticity stems from the excess of the neutrino emission parallel to the spin axis. This is because the compression of matter is more enhanced in the vicinity of the equatorial plane due to the growth of the spiral SASI modes, leading to the formation of spiral flows circulating around the spin axis with higher temperatures. We point out that a recently proposed future space interferometers like Fabry-Perot type DECIGO would permit detection of these signals for a Galactic supernova.

3D Core-Collapse Supernova Simulations: Neutron Star Kicks and Nickel Distribution

AbstractWe perform a set of neutrino-driven core-collapse supernova (CCSN) simulations studying the hydrodynamical neutron star kick mechanism in three-dimensions. Our simulations produce neutron star (NS) kick velocities in a range between ~100-600 km/s resulting mainly from the anisotropic gravitational tug by the asymmetric mass distribution behind the supernova shock. This stochastic kick mechanism suggests that a NS kick velocity of more than 1000 km/s may as well be possible. An enhanced production of heavy elements in the direction roughly opposite to the NS recoil direction is also observed as a result of the asymmetric explosion. This large scale asymmetry might be detectable and can be used to constrain the NS kick mechanism.

A MODEL FOR GRAVITATIONAL WAVE EMISSION FROM NEUTRINO-DRIVEN CORE-COLLAPSE SUPERNOVAE

Abridged: Using a suite of progenitor models, neutrino luminosities, and two- dimensional (2D) simulations, we investigate the matter gravitational-wave (GW) emission from postbounce phases of neutrino-driven core-collapse supernovae (CCSNe). The relevant phases are prompt and steady-state convection, the standing accretion shock instability (SASI), and asymmetric explosions. For the stages before explosion, we propose a model for the source of GW emission. Downdrafts of the postshock-convection/SASI region strike the protoneutron star "surface" with large speeds and are decelerated by buoyancy forces. We find that the GW amplitude is set by the magnitude of deceleration and, by extension, the downdraft's speed and the vigor of postshock-convective/SASI motions. However, the characteristic frequencies, which evolve from ~100 Hz after bounce to ~300-400 Hz, are primarily independent of these speeds, but are set by the deceleration timescale, which is in turn set by the buoyancy frequency at the lower boundary of postshock convection. Consequently, the GW characteristic frequencies are dependent upon a combination of core structure attributes, specifically the dense-matter equation of state (EOS) and details that determine the gradients at the boundary, including the accretion-rate history, the EOS at subnuclear densities, and neutrino transport. During explosion, the high frequency signal wanes and is replaced by a strong low frequency, ~10s of Hz, signal that reveals the general morphology of the explosion (i.e. prolate, oblate, or spherical). However, current and near-future GW detectors are sensitive to GW power at frequencies >50 Hz. Therefore, the signature of explosion will be the abrupt reduction of detectable GW emission.

Toward 2-D and 3-D simulations of core-collapse supernovae with magnetic fields

We describe a code development and integration effort aimed at producing a numerical tool suitable for exploring the effects of stellar rotation and magnetic fields in a neutrino-driven core-collapse supernova environment. A one-dimensional, multi-energy group, flux-limited neutrino diffusion module (MGFLD) has been integrated with ZEUS-MP, a multidimensional, parallel gas hydrodynamics and magnetohydrodynamics code. With the neutrino diffusion module, ZEUS-MP can simulate the core-collapse, bounce, and explosion of a stellar progenitor in two and three space dimensions in which multidimensional hydrodynamics are coupled to 1-D neutrino transport in a ray-by-ray approximation. This paper describes the physics capabilities of the code and the technique for implementing the serial MGFLD module for parallel execution in a multi-dimensional simulation. Because the development and debugging of the integrated code is not yet complete, we provide a current status report of the effort and identify outstanding issues currently under investigation.

Self‐shielded Downflows in Core‐Collapse Supernovae: Prospects for Laser Experiments

The core-collapse supernova mechanism pushes the frontiers of many aspects of physics and numerical modeling: equations of state for dense matter, neutrino cross sections and mixing, general relativity, and convection. The conditions in the convective layer driving the supernova explosion lead to both optically thin and optically thick regions. Thus, the convection is more complicated than the standard entropy-driven convection seen in a pot of boiling water. In this case, the convection is bathed in a radiation flow which heats the material and affects the convection. Radiation hydrodynamics, especially in the regime for which the optical depth is not extreme (τ ≈ 1), is not well understood. Fortunately, it may be possible to construct laboratory tests of this convection using laser experiments. In this paper, we summarize the neutrino-driven core-collapse supernova mechanism, describing its sensitivity to a range of physical parameters through a series of simulations. In particular, we notice that the varying results of the simulations may not be a difference in the neutrino heating rates alone, but rather a difference in the coupling of radiation with convection. This problem could be solved with an appropriately designed laser experiment.