Impact of 3D macro physics and nuclear physics on the p nuclei in O-C shell mergers

TL;DR

Massive-star O-C shell mergers create a convective-reactive γ-process environment in which p-nuclei are synthesized prior to core collapse. By post-processing a 1D NuGrid model with 3D-inspired mixing profiles, the authors quantify how downturns in velocity, enhanced mixing, C-shell ingestion, and convective quenching modify p-nuclei yields, finding non-linear responses and substantial isotopic shifts. They show that macrophysical uncertainties from 3D hydrodynamics can be as impactful as uncertainties in nuclear reaction rates, underscoring the need to incorporate realistic 3D mixing when predicting p-nuclei production. The work also provides correlations between reaction rates and abundances that depend on the mixing scenario, and discusses implications for presolar grains and galactic chemical evolution. Overall, the study highlights the critical role of 3D macro-physics in shaping the pre-explosion nucleosynthesis landscape of p nuclei in O-C shell mergers.

Abstract

O-C shell mergers in massive stars are a site for producing the p nuclei by the $γ$ process, but 1D stellar models rely on mixing length theory, which does not match the radial velocity profiles of 3D hydrodynamic simulations. We investigate how 3D macro physics informed mixing impacts the nucleosynthesis of p nuclei. We post-process the O-shell of the $M_\mathrm{ZAMS} = 15~\mathrm{M}_\odot$, $Z = 0.02$ model from the NuGrid stellar data set. Applying a downturn to velocities at the boundary and increasing velocities across the shell as obtained in previous results, we find non-linear, non-monotonic increase in p-nuclei production with a spread of 0.96 dex, and find that isotopic ratios can change. Reducing C-shell ingestion rates as found in 3D simulations suppresses production, with spreads of 1.22-1.84 dex across MLT and downturn scenarios. Applying dips to the diffusion profile to mimic quenching events also suppresses production, with a 0.51 dex spread. We analyze the impact of varying all photo-disintegration rates of unstable n-deficient isotopes from Se-Po by a factor of 10 up and down. The nuclear physics variations for the MLT and downturn cases have a spread of 0.56-0.78 dex. We also provide which reaction rates are correlated with the p nuclei, and find few correlations shared between mixing scenarios. Our results demonstrate that uncertainties in mixing arising from uncertain 3D macro physics are as significant as nuclear physics and are crucial for understanding p-nuclei production during O-C shell mergers quantitatively.

Figures (23)

• Figure 1: Kippenhahn diagram showing the merger of the convective O and $\mathrm{^{}C}$burning shells. The $\mathrm{^{}O}$burning shell extends from $1.55\;\mathrm{\mathrm{M}_\odot}$ to $1.95\;\mathrm{\mathrm{M}_\odot}$, and the first ingested $\mathrm{^{}C}$-burning shell from $1.96\;\mathrm{\mathrm{M}_\odot}$ to $2.11\;\mathrm{\mathrm{M}_\odot}$. A red guideline has been provided to mark the thin radiative layer separating the $\mathrm{^{}O}$ and $\mathrm{^{}C}$ shell. Other convective regions are also $\mathrm{^{}C}$burning shells. The merger onsets at $\log_{10}(t-t_{\mathrm{end}}) /\mathrm{yr} \approx -3.85$ and reaches full extent at $\approx-4$. A black triangle marks where the initial composition is taken from and a white star marks the location where the ingested $\mathrm{^{}C}$-shell material is taken from for this study.
• Figure 2: Mass fractions of the $p$ nuclei from $\log_{10}(t-t_{\mathrm{end}}) /\mathrm{yr}=-3.856$ used for initial $\mathrm{^{}O}$-shell composition and ingested $\mathrm{^{}C}$-shell material. Markers are the same as Figure \ref{['fig:kippenhahn']}.
• Figure 3: The diffusion coefficient profile and mixing length at model number 9200 for the $M_{\mathrm{ZAMS}}=15\;\mathrm{\mathrm{M}_\odot}$, $Z=0.02$ model. The light blue line is $D$ from MESA, the orange line the smoothed $D$ used for the MLT mixing scenario in this paper, and the grey line is the mixing length. Black dashed lines mark the shell boundaries for this paper.
• Figure 4: The diffusion coefficient profiles for the MLT and 3D-inspired gradual downturn scenarios. The dashed orange line is $D_{\mathrm{MLT}}$ and the dashed light grey line, dotted light blue line, solid grey line, and dashed dark blue line are the downturn profiles with boost factors of $1$, $3$, $10$, and $50$ respectively.
• Figure 5: The quenched mixing scenario convective profiles. The dashed light orange line and dotted light grey line are the GOSH-like profiles with a dip centred at $r=4.95\;\mathrm{\mathrm{M}\mathrm{m}}$. The solid red line and dashed dark grey line are the partial merger profiles with a dip centred $r=7.5~\mathrm{Mm}$.