The physics of deep-interior convective shell boundaries in a massive pre-supernova star: Internal gravity wave mixing at the N2 peak
Abstract
The physics of convective boundary mixing involves concepts such as penetration and overshooting and has been mostly characterized for envelope and core convection. For the burning shells deep inside massive stars in their final stages, the convective boundary remains poorly understood. 1D stellar-evolution codes typically place a steep entropy gradient and the corresponding narrow \brunt{} (N2) peak at a lower convective shell boundary. Based on high-resolution 3D hydrodynamic simulations of a 25 M⊙ star 11 days before core collapse, we find that convection excites high-frequency internal gravity waves at the N2 peak representing the C-shell convection bottom. These waves efficiently mix entropy and species and the N2 peak erodes by ∼40% within hours, correspondingly reducing the entropy gradient. The wave-driven diffusion coefficient is D≈ 5−−6× 109 cm2 second−1 at the N2 peak in numerically converged simulations. Our simulations show that the dominant physics at this boundary is mixing due to internal gravity waves. Extrapolated with the time-dependent D coefficient from the 3D simulations, the entropy step across the peak decreases by 40--85\% within half a day, thereby reducing the density, temperature and entropy gradients that set the accretion-rate drop a stalled supernova shock encounters. This physics is not included in present-day 1D models and enters the structural parameters that predict explodability. Flattening the gradients days before collapse favors an O--C shell merger, which could reshape pre-collapse nucleosynthesis. Without this physics 1D models carry too sharp gradients that introduce previously unaccounted systematic errors into the explodability surveys, yield tables and chemical-evolution studies built on them.
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BibTeX
@misc{pathak2026physics,
title = {{The physics of deep-interior convective shell boundaries in a massive pre-supernova star: Internal gravity wave mixing at the $N^2$ peak}},
author = {Praneet Pathak and Falk Herwig and Simon Blouin and Paul R. Woodward and Joshua Issa},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.28667},
eprint = {2609.28667},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.28667},
}
RIS
TY - UNPB
TI - The physics of deep-interior convective shell boundaries in a massive pre-supernova star: Internal gravity wave mixing at the N^2 peak
AU - Praneet Pathak
AU - Falk Herwig
AU - Simon Blouin
AU - Paul R. Woodward
AU - Joshua Issa
PY - 2026
DA - 2026/09/23
PB - arXiv
DO - 10.48550/arxiv.2609.28667
UR - https://arxiv.org/abs/2609.28667
AB - The physics of convective boundary mixing involves concepts such as penetration and overshooting and has been mostly characterized for envelope and core convection. For the burning shells deep inside massive stars in their final stages, the convective boundary remains poorly understood. 1D stellar-evolution codes typically place a steep entropy gradient and the corresponding narrow \brunt{} (N^2) peak at a lower convective shell boundary. Based on high-resolution 3D hydrodynamic simulations of a 25 M_⊙ star 11 days before core collapse, we find that convection excites high-frequency internal gravity waves at the N^2 peak representing the C-shell convection bottom. These waves efficiently mix entropy and species and the N^2 peak erodes by ∼40% within hours, correspondingly reducing the entropy gradient. The wave-driven diffusion coefficient is D≈ 5−−6× 10^9 cm^2 second^−1 at the N^2 peak in numerically converged simulations. Our simulations show that the dominant physics at this boundary is mixing due to internal gravity waves. Extrapolated with the time-dependent D coefficient from the 3D simulations, the entropy step across the peak decreases by 40--85\% within half a day, thereby reducing the density, temperature and entropy gradients that set the accretion-rate drop a stalled supernova shock encounters. This physics is not included in present-day 1D models and enters the structural parameters that predict explodability. Flattening the gradients days before collapse favors an O--C shell merger, which could reshape pre-collapse nucleosynthesis. Without this physics 1D models carry too sharp gradients that introduce previously unaccounted systematic errors into the explodability surveys, yield tables and chemical-evolution studies built on them.
ER -