PaperlayerView exact revision on arXiv

Source-linked AI summary

Explosion Mechanisms of Core-Collapse Supernovae

H. -Thomas Janka

arXiv:1206.2503v1astro-ph.SRgr-qchep-phnucl-th

TL;DR

The review asks how improved simulations and physics explain core-collapse explosions and their observable consequences. It synthesizes advances in neutrino transport, microphysics, and multidimensional hydrodynamics, finding that neutrino heating aided by nonradial flows drives some low-energy explosions, whereas the most energetic events likely require magnetorotational driving.

  • Problem

    The paper addresses how stellar cores explode, how explosion mechanisms shape observable signals and nucleosynthesis, and whether neutrino-powered models can account for the full range of supernova energies.

  • Method

    The review combines developments in multidimensional simulations, neutrino transport, microphysics, hydrodynamic-instability analysis, and observational diagnostics.

  • Results

    Neutrino heating aided by nonradial flows produces low-energy explosions in ONeMg-core and some Fe-core progenitors, but neutrino-driven models are unlikely to explain energies above ∼2 × 10^51 erg, motivating alternative engines.

  • Takeaways & Limitations

    Nonradial instabilities help revive stalled shocks and explain large pulsar kicks and mixing, while hyperenergetic explosions require mechanisms beyond ordinary neutrino heating.

Abstract

from arXiv · show

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.

I. INTRODUCTION: ROOTS AND QUESTIONS

Core-collapse supernovae arise from several stellar-evolution pathways and raise interconnected questions about explosions, remnants, signals, nucleosynthesis, and galactic impact. The review updates numerical and analytic understanding of these mechanisms, emphasizing neutrino heating aided by multidimensional flows.

  • Roots and motivation: The central problem is determining when, why, and how massive-star core infall reverses into an explosion that ejects the stellar mantle and synthesizes and disperses heavy elements.The question connects supernova physics with nuclear, particle, and gravitational physics.
  • Explosion mechanism: The delayed neutrino-heating mechanism, aided by violent nonradial mass motions, is the favored scenario for powering most core-collapse supernovae.Modern simulations do not support the bounce-shock mechanism as the general explosion mechanism.
  • Open questions: The review addresses progenitor links, compact-remnant properties, pulsar velocities, neutrino and gravitational-wave signals, nucleosynthesis, r-process production, and galactic energetic impact.These questions motivate the review's treatment of collapse events, modeling, explosion mechanisms, and observational diagnostics.
  • Review scope: The article reviews collapse events, numerical-modeling ingredients, candidate explosion mechanisms, and observable signatures as an update and supplement to earlier reports [14] [15] [16] [17].Its scope spans stellar evolution through explosion diagnostics.
  • Routes to collapse: Core-collapse outcomes depend on stellar evolution, with collapse initiated when the effective adiabatic index falls below 4/3 in gravitationally unstable death zones.Three distinct instability processes operate in different regions of the central-density–temperature plane.

A. Electron-Capture Supernovae

Electron-capture supernovae arise from low-mass progenitors with oxygen-neon-magnesium cores that become electron-degenerate before neon burning. Their steep edge density gradient produces faint explosions with little carbon, oxygen, and nickel, consistent with some observed dim transients.

  • Progenitors: Electron captures trigger collapse in low-mass ONeMg-core stars after electron degeneracy prevents hydrostatic neon burning; solar-metallicity progenitors are estimated near 9–9.25 M⊙.The mass window may shift and widen at lower metallicity and in binaries with mass loss.
  • Explosion properties: A steep density decline at the ONeMg-core edge yields little carbon and oxygen ejection and very little nickel, producing relatively faint supernovae.The thin C-O shell spans roughly 0.1 M⊙ between about 3 × 10^4 and 4 × 10^8 g/cm^3.
  • Observational context: Theoretical uncertainties produce substantially different inferred progenitor masses for several observed explosions, including SN 1999br and SN 2005cs.Figure 3 compares kinetic energies and ejected nickel masses against estimated ZAMS masses from different methods.

B. Iron-Core Supernovae

Iron-core collapse begins when photodisintegration and electron captures destabilize the core, but flatter density profiles sustain accretion that inhibits shock expansion. Ordinary neutrino heating may explain some low-energy events, whereas hyperenergetic explosions require alternative rotational or other engines.

  • Collapse physics: Iron-core collapse is accelerated by iron-group photodisintegration and electron captures, then halted at nuclear density when the inner core rebounds and launches a shock.The shock initially encounters infalling layers whose ram pressure affects whether it can produce an explosion.
  • Shock revival: Flatter Fe-core density profiles sustain high accretion rates and ram pressure, making Fe-core progenitors harder to explode than ONeMg-core stars.The mechanism of successful Fe-core explosions remains under discussion.
  • Hypernovae and collapsars: Rapid rotation is expected to be crucial for gamma-ray-burst supernovae and hypernovae, whose broad lines indicate unusually high ejecta velocities and kinetic energies.Collapsar models require compact, rapidly rotating progenitors capable of producing relativistic jets.
  • Pair instability: Pair instability can cause collapse or complete thermonuclear disruption: intermediate masses may produce explosions above 10^53 erg and up to ≳50 M⊙ of ^56Ni [44].Stars between roughly 100 and 140 M⊙ and above about 260 M⊙ are expected to collapse to black holes, while intermediate masses can disrupt completely.
  • Scope boundary: Ultra-bright transients have uncertain origins, with proposed explanations including pair-instability explosions, circumstellar interaction, and magnetar spin-down.The review therefore focuses mainly on physics relevant to ordinary core-collapse supernovae.

III. NUMERICAL MODELING AND PHYSICS INGREDIENTS

Supernova modeling combines multidimensional hydrodynamics, gravity, nuclear equations of state, and increasingly sophisticated neutrino transport, but fully realistic 3D calculations remain computationally and methodologically constrained. Equation-of-state choices and neutrino-transport approximations can materially affect postbounce evolution, while direct multidimensional code comparisons are still lacking.

  • III. NUMERICAL MODELING AND PHYSICS INGREDIENTS: Modern modeling incorporates multidimensional fluid instabilities, general relativistic gravity, finite-temperature nuclear equations of state, and energy-dependent neutrino transport.State-of-the-art transport includes three-flavor, multigroup treatments with energy-bin coupling and velocity-dependent corrections in 1D, while multidimensional implementations remain more varied.
  • III. NUMERICAL MODELING AND PHYSICS INGREDIENTS: Three-dimensional simulations remain limited because fully realistic neutrino-hydrodynamics requires extreme computational resources and current calculations still simplify important physics.A single 3D model can require weeks to months, while available self-consistent calculations have sacrificed quantitative aspects such as general relativity or detailed neutrino transport.
  • B. Neutrino Transport: Neutrino transport methods differ substantially in multidimensional simulations, including MGFLD, multi-angle solvers, two-moment closures, and IDSA, with some omitting energy coupling or fluid-motion corrections.The IDSA decomposes the neutrino distribution into trapped and streaming components coupled through a diffusion source term.
  • B. Neutrino Transport: FLD underestimates angular neutrino variations whereas ray-by-ray sharpens them, although reported hydrodynamic consequences of these differences can be limited by variable nonspherical accretion flows.Direct comparisons among multidimensional codes and approximations have not yet been performed, unlike in 1D.
  • C. Equation of State and Composition of Stellar Plasma: Equation-of-state differences produce modest changes during collapse and early postbounce evolution in 1D but can strongly influence multidimensional explosions through proto-neutron-star contraction.A more rapidly shrinking proto-neutron star emits neutrinos with higher fluxes and energies, enhancing heating and hydrodynamic instabilities; the LS180-EoS also conflicts with the observed 1.97 M⊙ pulsar constraint.

IV. EXPLOSION MECHANISMS

Core-collapse explosions require transferring energy from a nascent neutron star to overlying layers. Neutrino heating can power low-energy explosions, whereas the highest energies suggest magnetorotational driving.

  • Neutrino heating is self-regulated because heated matter expands away after acquiring energy comparable to its binding energy, limiting further energy deposition.
  • Neutrino-driven Fe-core explosions develop violent convection, SASI sloshing, and strong asymmetries, including unipolar shock expansion in a 15 M⊙ model.
  • Core-collapse supernovae have explosion energies from ≳10^50 erg to several 10^52 erg for hypernovae, with the highest energies suggesting a mechanism beyond neutrino heating.
  • Magnetorotational explosions can supply unusually large energies by extracting rotational energy from a rapidly spinning protoneutron star through magnetic fields.
  • The proposed neutrino-triggered thermonuclear scenario for low-mass progenitors is not supported because detailed progenitor and explosion models do not realize its assumed conditions.
  • Presently, pair-instability supernovae are the only stellar core-collapse events known to rely on thermonuclear energy release.

B. Bounce-Shock Mechanism

The prompt bounce shock cannot explode any progenitor in modern simulations. Shock stagnation instead involves accretion, neutrino cooling and heating, and hydrodynamic instabilities that help determine whether an explosion develops.

  • B. Bounce-Shock Mechanism: Modern analytical studies and simulations agree that the prompt bounce-shock mechanism cannot cause an explosion for any progenitor star.
  • B. Bounce-Shock Mechanism: Improved electron-capture microphysics reduces the homologous inner-core mass, while nuclear dissociation and weak rebound rapidly drain the bounce shock's energy.
  • B. Bounce-Shock Mechanism: The shock continues moving outward despite downstream accretion because accreted matter radiates neutrinos and gradually increases postshock pressure.
  • B. Bounce-Shock Mechanism: After roughly 100 ms of slow expansion, a gain radius forms where neutrino cooling inside changes to neutrino heating outside.
  • B. Bounce-Shock Mechanism: Neutrino heating creates a negative entropy gradient, but convection in the infalling postshock flow requires growth or buoyancy times shorter than advection.
  • B. Bounce-Shock Mechanism: Hydrodynamic instabilities generically break spherical symmetry and are decisive for the success of neutrino heating and the stalled shock's later evolution.

1. Heating Efficiency and Energetics

Neutrino heating depends on absorption in the gain layer and on how long matter remains exposed to the radiation. Typical accretion conditions yield heating rates of about 10^51–10^52 erg/s, moderated by cooling losses.

  • 1. Heating Efficiency and Energetics: The absorption estimate uses the neutrino optical depth through the gain layer, based on the density jump, preshock free-fall flow, and mass-accretion rate.
  • 1. Heating Efficiency and Energetics: For accretion rates of 0.1–0.5 M⊙/s, several percent of neutrino luminosity can be absorbed in the gain layer, producing Q+_ν ≈10^51–10^52 erg/s.
  • 1. Heating Efficiency and Energetics: The gain-layer optical depth is not a complete measure of heating efficiency because moving matter has finite residence times and multidimensional inflows and outflows.
  • 1. Heating Efficiency and Energetics: Matter near the gain radius can absorb about 50 MeV per nucleon over 0.1 s under representative 4 MeV neutrino conditions.
  • 1. Heating Efficiency and Energetics: The estimated heating figures are compatible with detailed simulations after reducing them by 20–30% for neutrino-cooling losses.

2. Hydrodynamical Explosion Models

Neutrino heating succeeds most clearly for ONeMg-core progenitors and some Fe-core stars when multidimensional flows aid energy deposition, but outcomes remain method-sensitive and instability physics unsettled.

  • ONeMg-core explosions: EONeMg ≈1050 erg was obtained for an 8.8 M⊙ ONeMg-core explosion, with at most ∼10% enhancement in 2D and little nickel ejection.The explosion energy and several 10^-3 M⊙ of nickel are compatible with estimates for the Crab supernova.
  • Fe-core explosions: 11.2 and 15 M⊙ Fe-core stars produced weak neutrino-driven explosions in 2D, whereas corresponding 1D models did not explode.Convective overturn combined with SASI activity was decisive for the multidimensional success.
  • Model dependence and open questions: Different multidimensional studies disagree on explosion success, highlighting strong sensitivity to transport, microphysics, numerics, and the equation of state.The exact roles of SASI, hydrodynamic instabilities, and turbulent motions in triggering explosion remain under active investigation.
  • Effects of nonspherical flows: Nonradial turbulent flows increase gain-layer residence time and mass, raising neutrino energy deposition and integrated energy transfer.Rayleigh-Taylor structures channel cool material toward the gain radius while buoyant bubbles reduce reemission losses.
  • Effects of nonspherical flows: SASI activity expands the shock, creates secondary shocks that dissipate kinetic energy, and strengthens convection through additional heating.After explosion onset, simultaneous shock expansion and accretion sustain higher neutrino fluxes and heating than in 1D.

4. Runaway Threshold

Explosion onset is associated with a global runaway of the neutrino-heated postshock layer above a critical threshold, while multidimensional effects lower that threshold and magnetorotational alternatives address more energetic explosions.

  • Analytic threshold: The steady-state critical luminosity Lν,c(Ṁ) is close to, but not identical with, the runaway threshold defined by t_adv > t_heat.The analytic scaling reproduces the functional behavior of the stationary-accretion result, with a numerical factor of (5–6)×10^52 erg/s for representative parameters.
  • Multidimensional effects: For fixed accretion rate, multidimensional simulations lower the critical neutrino luminosity by typically several 10%, although the underlying reasons remain unsettled.The improvement has been found in both stationary-flow analyses and time-dependent collapsing-core calculations.
  • Runaway threshold: The explosion threshold is linked to a global runaway of the postshock accretion layer fueled by neutrino deposition above a critical level.Analytic and simulation studies frame the key condition as t_adv/t_heat > 1.
  • Magnetorotational mechanism: Magnetorotational explosions require very rapid core rotation, with simulations suggesting P_core ≲2–5 s, whereas stellar-evolution models generally predict slower cores.Magnetic amplification can proceed through field winding or MRI growth, potentially reaching 10^15–10^16 G under sufficiently rapid rotation.
  • Magnetorotational mechanism: MHD explosions can exceed neutrino-driven energies, whose blast-wave energies may be limited to ∼(1–2)×10^51 erg, and are expected to produce global deformation and collimated jets.Reliable predictions remain limited because realistic MHD requires 3D modeling and depends strongly on initial rotation and magnetic-field conditions.

E. Acoustic Mechanism

The acoustic mechanism proposes that SASI and anisotropic accretion excite large-amplitude PNS g-modes whose sound waves transfer accretion energy to the surrounding medium, but its efficiency and final explosion energies remain uncertain.

  • Acoustic mechanism: Late-time SASI sloshing and anisotropic accretion can excite large-amplitude dipole g-mode oscillations of the PNS core.The simulated oscillations had amplitudes of several kilometers and were damped by strong sound waves sent into the surrounding medium.
  • Acoustic energy transfer: The vibrating PNS can act as a transducer, converting accretion energy into sound whose estimated power exceeded late-time neutrino energy deposition.The acoustic power estimate depends on the accretion-stream radius, wave height, PNS-core density, and surface gravity.
  • Limitations: The accretion-to-g-mode conversion fraction and final explosion energies could not be reliably determined, while the 2D explosions occurred very late and tended to be low-energy.The excitation efficiency of the large-amplitude, low-order PNS g-modes remains a fundamental open question.

F. Phase-Transition Mechanism

A tuned hadron-to-quark phase transition can destabilize the PNS and trigger a second collapse, while the resulting neutrino signals and multidimensional dynamics provide diagnostics of core evolution and explosion conditions.

  • Phase-transition mechanism: A sufficiently low-density first-order hadron-to-quark transition can destabilize the PNS and trigger a second supersonic collapse followed by renewed energy release.The mechanism relies on a soft mixed-phase equation of state and subsequent stiffening in the pure quark phase.
  • Phase-transition limitations: The phase-transition explosion scenario is constrained because successful equations of state so far conflict with the 1.97±0.04M⊙ mass limit of PSR J1614-2230.Whether compatible deconfinement equations of state can produce explosions remains unresolved.
  • Neutrino diagnostics: Neutrino signals encode thermodynamic and dynamical conditions in the forming neutron star and could help distinguish explosion mechanisms in a future Galactic event.Shock-breakout bursts, phase-transition flashes, and heavy-lepton mean-energy rises are examples of diagnostic features.
  • Neutrino diagnostics: SASI and convective activity modulate accretion luminosities and mean energies, with relative hemispheric differences revealing variations that directional integration can obscure.Such modulation may be detectable with IceCube or future megaton-class detectors, although first 3D estimates suggest smaller amplitudes than in 2D.
  • Neutrino spectra: State-of-the-art models find ⟨εν̄e⟩ approximately equal to ⟨ενx⟩ and ⟨ενx⟩ ≲13–16 MeV, replacing the earlier expectation of a strongly hotter νx spectrum.The ordering varies with time and equation of state; during late cooling, all neutrino mean energies become nearly identical.

B. Gravitational Waves

Core-collapse supernova motions generate gravitational waves whose signals track convection, SASI activity, asymmetric neutrino emission, and post-explosion accretion. These signatures may distinguish explosion mechanisms, but detailed 3D waveforms vary strongly with viewing direction.

  • Nonradial motions source gravitational waves, with amplitudes proportional to the second time derivative of the mass-quadrupole moment.The signal partly mirrors activity phases visible in neutrino-luminosity variations.
  • Neutrino-driven explosions produce broadband gravitational-wave activity from convection and SASI, whereas acoustic explosions would show a sharp rise from large-amplitude PNS g-mode oscillations.
  • The same broad gravitational-wave activity phases occur in 3D, but amplitudes and detailed signal structure vary strongly with observer direction and lack a template character.
  • The matter signal records prompt convection, increasingly violent convective and SASI motions before shock revival, and asymmetric accretion downdrafts after explosion.In the 11.2 and 15 M⊙ models, pre-explosion activity extends to roughly 350 ms and 500 ms, respectively.

VI. EXPLOSION PROPERTIES AND COMPACT REMNANTS

Explosion asymmetries can produce large neutron-star kicks through sustained gravitational tugging, while hydrodynamic accretion can also redistribute angular momentum. Current simulations indicate that such flows generate spins slower than observed millisecond birth periods.

  • A 1% anisotropy in total neutrino energy loss could produce a kick exceeding 300 km/s, but the associated emission is highly time-variable and nonstationary.
  • Asymmetric ejecta can gravitationally accelerate a neutron star for seconds, reaching velocities of many 100 km/s and, for strong asymmetries, above 1000 km/s.This long-time pull supplements the shorter hydrodynamic contact force during explosion launch.
  • Anisotropic mass ejection links neutron-star recoil to the distribution of heavy elements because slower, denser ejecta exert the strongest gravitational pull.
  • Hydrodynamic flows and post-explosion accretion yield neutron-star spin periods of hundreds of milliseconds to seconds, insufficient for estimated birth periods of about 10 ms.Rapid rotation of the collapsing stellar core therefore appears necessary for the shortest inferred birth spins.

B. Supernova Asymmetries

Core instabilities seed asymmetric ejecta and mixing, while progenitor structure produces strongly nonmonotonic explosion and remnant properties. Parametrized neutrino-driven models reproduce some remnant trends but appear unable to reach hypernova-scale energies and nickel yields.

  • B. Supernova Asymmetries: Large core asymmetries seed Rayleigh–Taylor mixing, allowing ^56Ni-rich clumps to cross the hydrogen envelope at speeds up to 4500 km/s in SN 1987A models.This reproduces early radioactive X-ray and γ-ray signals better than one-dimensional models.
  • C. Neutron Stars and Black Holes: Explosion properties vary strongly with stellar structure, with large differences possible between progenitors separated by only a narrow mass interval.
  • C. Neutron Stars and Black Holes: Failed explosions can form black holes below 20 M⊙, while successful neutron-star-forming explosions also occur between 20 and 40 M⊙.
  • C. Neutron Stars and Black Holes: Neutrino-driven explosions are unlikely to exceed 2 × 10^51 erg or produce substantially more than 0.1 M⊙ of ^56Ni.This limitation supports magnetorotational driving for events with energies from several 10^51 erg to above 10^52 erg.
  • C. Neutron Stars and Black Holes: Explosion onset spans roughly 0.1–1.1 s after bounce, and later explosions tend to be less energetic because less mass remains available for neutrino heating.
  • C. Neutron Stars and Black Holes: The enclosed mass at the base of the oxygen-burning shell is not a reliable predictor of stellar fate, because some relatively small silicon cores still fail to explode.

VII. SUMMARY, CONCLUSIONS, OUTLOOK

Neutrino heating, aided by nonradial flows, can produce low-energy explosions and explain several asymmetry and remnant signatures. Its apparent energy and nickel limits point toward magnetorotational engines for hyperenergetic events, while 3D modeling and improved observations remain essential.

  • Neutrino-driven models reproduce low-energy explosions of ONeMg-core progenitors and some Fe-core stars, while simulations suggest energies above about 2 × 10^51 erg require another engine.
  • Nonradial flows lower the neutrino-luminosity threshold for runaway by lengthening gain-region residence times and improving neutrino-energy deposition.The exact fastest-growing instability mode remains under debate and requires further 3D study.
  • Hydrodynamic instabilities generate low-multipole asymmetries that can explain pulsar kicks above 1000 km/s and seed mixing of inner-core material into outer ejecta.
  • Systematic progenitor studies find strong sensitivity of explosion properties to stellar structure, including large variations within narrow progenitor-mass intervals.
  • Reliable predictions remain limited by uncertain multidimensional progenitor structure, rotation, and magnetic fields at core collapse.
Loading 1206.2503v1…