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3D Macro Physics and Light Odd-Z Element Production in O-C Shell Mergers: Implications for $^{40}\mathrm{K}$ production and radiogenic heating inventories of rocky exoplanets

Joshua Issa, Falk Herwig, Stephen J. Mojzsis, Marco Pignatari

TL;DR

The paper investigates how convective macro-physics in 3D O-C shell mergers of massive stars alters the pre-explosive production of light odd-Z elements (P, Cl, K, Sc) and, in particular, the radiogenic nuclide $^{40}$K that powers early rocky-planet heating. Using a NuGrid $15 M_\\odot$, $Z=0.02$ baseline model, the authors post-process 24 mixing scenarios (4 MLT, 16 3D-inspired with a convective downturn and boosted velocities, plus 4 quenched cases) with a 1470-isotope network over ~110 s to quantify how 3D macro-physics changes yields and reaction pathways. They find substantial, non-monotonic variations in light odd-Z production, with elemental K showing the largest sensitivity (up to ~38x increase) and $^{40}$K yields spanning more than three orders of magnitude across mixing cases; this implies strong heterogeneity in ISM $^{40}$K enrichment and significant implications for rocky exoplanet radiogenic heating histories. The study highlights the need for fully 3D hydrodynamic simulations to constrain mixing during shell mergers, as 1D treatments cannot capture the observed variability and its consequences for galactic chemical evolution and planetary evolution.

Abstract

The light odd-Z elements P, Cl, K, and Sc are underproduced in galactic chemical evolution models compared to spectroscopic observations of stars in the Milky Way. The most promising solution to this puzzle is that some massive stars experience O-C shell mergers boosting their yields through dynamic, convective-reactive nucleosynthesis. We report how convective macro physics based on 3D $4π$ hydrodynamic simulations impacts production in the O shell by post-processing the $\mathrm{M_{ZAMS}}=15~\mathrm{M_\odot}$ $Z=0.02$ model from the NuGrid dataset. We explore a mixing downturn, boosted velocities, reduced ingestion rate, and convective quenching. Across 24 mixing cases, the pre-explosive yields for [P/Fe], [Cl/Fe], [K/Fe], and [Sc/Fe] are modified by $[-0.33,0.23]~\mathrm{dex}$, $[-0.84,0.64]~\mathrm{dex}$, $[-0.78,1.48]~\mathrm{dex}$, and $[-0.36,1.29]~\mathrm{dex}$, respectively. Cases with a convective downturn with the fastest ingestion rate have the largest enhancement, and production is non-monotonic with boosted velocities. Which reactions are most important for the convective-reactive element production pathways depends on the mixing. We parameterize production of $^{40}\mathrm{K}$ ($t_{1/2} = 1.248~\mathrm{Gyr}$), an important radiogenic heat source for younger ($2{-}3~\mathrm{Gyr}$) rocky planets and find a yield variation exceeding three orders of magnitude. This range of initial abundances for $^{40}\mathrm{K}$ implies the early geodynamic behaviour of silicate mantles in rocky planets can differ greatly from that of Earth. These results underscore the importance of investigating the 3D macro physics of shell merger convection through hydrodynamic simulations to develop a predictive understanding of the origin and variability of the light odd-Z elements and the $^{40}\mathrm{K}/\mathrm{K}$ ratio in planet host stars.

3D Macro Physics and Light Odd-Z Element Production in O-C Shell Mergers: Implications for $^{40}\mathrm{K}$ production and radiogenic heating inventories of rocky exoplanets

TL;DR

The paper investigates how convective macro-physics in 3D O-C shell mergers of massive stars alters the pre-explosive production of light odd-Z elements (P, Cl, K, Sc) and, in particular, the radiogenic nuclide K that powers early rocky-planet heating. Using a NuGrid , baseline model, the authors post-process 24 mixing scenarios (4 MLT, 16 3D-inspired with a convective downturn and boosted velocities, plus 4 quenched cases) with a 1470-isotope network over ~110 s to quantify how 3D macro-physics changes yields and reaction pathways. They find substantial, non-monotonic variations in light odd-Z production, with elemental K showing the largest sensitivity (up to ~38x increase) and K yields spanning more than three orders of magnitude across mixing cases; this implies strong heterogeneity in ISM K enrichment and significant implications for rocky exoplanet radiogenic heating histories. The study highlights the need for fully 3D hydrodynamic simulations to constrain mixing during shell mergers, as 1D treatments cannot capture the observed variability and its consequences for galactic chemical evolution and planetary evolution.

Abstract

The light odd-Z elements P, Cl, K, and Sc are underproduced in galactic chemical evolution models compared to spectroscopic observations of stars in the Milky Way. The most promising solution to this puzzle is that some massive stars experience O-C shell mergers boosting their yields through dynamic, convective-reactive nucleosynthesis. We report how convective macro physics based on 3D hydrodynamic simulations impacts production in the O shell by post-processing the model from the NuGrid dataset. We explore a mixing downturn, boosted velocities, reduced ingestion rate, and convective quenching. Across 24 mixing cases, the pre-explosive yields for [P/Fe], [Cl/Fe], [K/Fe], and [Sc/Fe] are modified by , , , and , respectively. Cases with a convective downturn with the fastest ingestion rate have the largest enhancement, and production is non-monotonic with boosted velocities. Which reactions are most important for the convective-reactive element production pathways depends on the mixing. We parameterize production of (), an important radiogenic heat source for younger () rocky planets and find a yield variation exceeding three orders of magnitude. This range of initial abundances for implies the early geodynamic behaviour of silicate mantles in rocky planets can differ greatly from that of Earth. These results underscore the importance of investigating the 3D macro physics of shell merger convection through hydrodynamic simulations to develop a predictive understanding of the origin and variability of the light odd-Z elements and the ratio in planet host stars.

Paper Structure

This paper contains 11 sections, 7 equations, 12 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Methodology
  3. The O-C shell merger model
  4. Post-Processed Models
  5. Estimating Pre-Explosive Yields
  6. Estimating Reaction Contribution
  7. Results
  8. Nucleosynthesis of the light odd-Z isotopes
  9. Impact of macro physics mixing on light odd-Z nucleosynthesis production
  10. Heterogeneous $\mathrm{^{40}K}$ interstellar medium enrichment and impact for rocky planet heating
  11. Discussions and Conclusions

Figures (12)

  • Figure 1: Chart of reactions between isotopes at $m = 1.65\;\mathrm{\mathrm{M}_\odot}$ [$T = 2.231\;\mathrm{\mathrm{G}\mathrm{K}}$] for the MLT mixing case with an ingestion rate of $4\times 10^{-3}\;\mathrm{\mathrm{M}_\odot\mathrm{s}^{-1}}$ at $t=110\;\mathrm{\mathrm{s}}$. Both arrow colour and size indicate $\log_{10}(f_{ij})$, and arrows point in the direction of the reaction.
  • Figure 2: Isotopic mass fractions at $t=110\;\mathrm{\mathrm{s}}$ without decays for the MLT mixing case with an ingestion rate of $4\times 10^{-3}\;\mathrm{\mathrm{M}_\odot\mathrm{s}^{-1}}$.
  • Figure 3: Overproduction of the light odd-Z isotopes for all 24 mixing cases in the $\mathrm{^{}O}$ shell at $110\;\mathrm{\mathrm{s}}$. The average spread $\mathrm{OP}_{\mathrm{max}} - \mathrm{OP}_{\mathrm{min}} = 1.82\;\mathrm{\, \mathrm{dex}}$, and the line at $\mathrm{OP}=0$ is the initial amount.
  • Figure 4: The change in $\, \mathrm{dex}$ for [$\mathrm{^{}P}$/$\mathrm{^{}Fe}$] to the $M_{\rm ZAMS}=15\;\mathrm{\mathrm{M}_\odot}$$Z=0.02$ model from ritterNuGridStellarData2018 as described in §\ref{['sec:estimate_production']}. The lower x-axis is the ingestion rate in $\mathrm{M}_\odot\mathrm{s}^{-1}$ for the MLT (circles) and 3D-inspired mixing (squares) cases. The upper x-axis is the centre of the mixing efficiency dip in $\mathrm{M}$$\mathrm{m}$ and the extent of the dip in $\mathrm{c}\mathrm{m}^{2}\mathrm{s}^{-1}$ for the quenched (diamonds) mixing cases. Size indicates distance from $[\mathrm{^{}P}/\mathrm{^{}Fe}]_{\mathrm{R}}=0$, and colour indicates the magnitude. The ritterNuGridStellarData2018 full delayed pre-explosive yield $[\mathrm{^{}P}/\mathrm{^{}Fe}]_\odot = 2.13\;\mathrm{\, \mathrm{dex}}$, and the contributions just from winds are $-0.04\;\mathrm{\, \mathrm{dex}}$
  • Figure 5: The same as Fig. \ref{['fig:ratio_p']} but for $[\mathrm{^{}Cl}/\mathrm{^{}Fe}]_\mathrm{R}$. The ritterNuGridStellarData2018 full delayed pre-explosive yield $[\mathrm{^{}Cl}/\mathrm{^{}Fe}]_\odot = 2.14\;\mathrm{\, \mathrm{dex}}$, and the contributions just from winds are $0.21\;\mathrm{\, \mathrm{dex}}$
  • ...and 7 more figures