[Paper Review] Multidimensional supernova simulations with approximative neutrino transport. II. Convection and the advective-acoustic cycle in the supernova core

Leonhard Scheck, Hans‐Thomas Janka+2 more|ArXiv.org|Apr 23, 2007

Astrophysics and Cosmic Phenomena53 references109 citations

TL;DR

This study investigates the interplay between convection and the standing accretion shock instability (SASI) in 2D multidimensional supernova simulations with approximative neutrino transport. It demonstrates that SASI oscillations can trigger convective instability, and both instabilities jointly enhance neutrino energy deposition by prolonging matter advection time, thereby supporting the neutrino-driven explosion mechanism. The key finding is that the SASI growth is best explained by an advective-acoustic cycle (AAC) rather than a purely acoustic mechanism.

ABSTRACT

By 2D hydrodynamic simulations including a detailed equation of state and neutrino transport, we investigate the interplay between different non-radial hydrodynamic instabilities that play a role during the postbounce accretion phase of collapsing stellar cores. The convective mode of instability, which is driven by negative entropy gradients caused by neutrino heating or by time variations of the shock strength, can be identified clearly by the development of typical Rayleigh-Taylor mushrooms. However, in cases where the gas in the postshock region is rapidly advected towards the gain radius, the growth of such a buoyancy instability can be suppressed. In such a situation the shocked flow nevertheless can develop non-radial asymmetry with an oscillatory growth of the amplitude. This phenomenon has been termed ``standing accretion shock instability'' (SASI). It is shown here that the SASI oscillations can trigger convective instability and like the latter they lead to an increase of the average shock radius and of the mass in the gain layer. Both hydrodynamic instabilities in combination stretch the advection time of matter through the neutrino-heating layer and thus enhance the neutrino energy deposition in support of the neutrino-driven explosion mechanism. A rapidly contracting and more compact nascent NS turns out to be favorable for explosions, because the accretion luminosity and neutrino heating are larger and the growth rate of the SASI is higher. Moreover, we show that the oscillation period of the SASI and a variety of other features in our simulations agree with estimates for the advective-acoustic cycle (AAC), in which perturbations are carried by the accretion flow from the shock to the neutron star and pressure waves close an amplifying global feedback loop. (abridged)

Motivation & Objective

• Investigate the role of non-radial hydrodynamic instabilities—specifically convection and SASI—in the postbounce phase of core-collapse supernovae.
• Determine whether convection or the standing accretion shock instability (SASI) dominates in triggering explosion dynamics.
• Assess the influence of initial seed perturbations on the development of asymmetries and explosion onset.
• Evaluate the physical mechanism driving SASI growth, contrasting the advective-acoustic cycle (AAC) hypothesis with purely acoustic instability models.
• Understand how combined convection and SASI stretch advection time, enhancing neutrino energy deposition critical for explosion success.

Proposed method

• Perform 2D hydrodynamic simulations with a detailed equation of state for stellar plasma and approximative neutrino transport.
• Use a full 180° angular grid to allow for global (dipolar and quadrupolar) asymmetries, avoiding artificial constraints.
• Implement a neutrino heating and energy deposition model based on neutrino transport approximations to simulate postshock dynamics.
• Analyze the growth of instabilities via Rayleigh-Taylor mushroom structures (for convection) and oscillatory shock asymmetries (for SASI).
• Compare observed SASI oscillation periods and amplification factors with predictions from the advective-acoustic cycle (AAC) model.
• Test the consistency of SASI behavior with AAC theory by examining wave propagation paths, velocity gradients, and deceleration regions in the postshock flow.

Experimental results

Research questions

• RQ1 How do convection and SASI interact nonlinearly to enhance neutrino energy deposition in the postshock region?
• RQ2 What determines whether convection or SASI dominates the early postbounce dynamics in multidimensional simulations?
• RQ3 Is the growth of the SASI driven primarily by an advective-acoustic cycle (AAC) or by a purely acoustic instability mechanism?
• RQ4 How does the advection time of matter through the gain layer influence the efficiency of the neutrino-driven explosion mechanism?
• RQ5 To what extent do initial seed perturbations affect the final explosion properties, such as explosion energy or neutron star kick velocity?

Key findings

• Convection and SASI coexist and mutually enhance each other in the postshock region, increasing the average shock radius and gain layer mass.
• The oscillation period of the SASI in simulations matches the advection time from the shock to the radius of strongest deceleration (R∇), supporting the AAC hypothesis.
• SASI amplification factors and acoustic structure correlate strongly with velocity gradients in the postshock layer, consistent with AAC predictions.
• Stronger neutrino heating increases SASI amplification efficiency, a behavior explainable within the AAC framework but not by purely acoustic models.
• The observed SASI wave propagation paths are inconsistent with a single acoustic path, suggesting a feedback loop involving advected perturbations and acoustic waves—characteristic of AAC.
• The nonlinear interaction between convection and SASI stretches the advection time of accreted matter through the gain layer, thereby enhancing neutrino energy deposition and supporting explosion viability.