Proton-rich production of lanthanides: the νi process

TL;DR

The paper addresses the open question of lanthanide origins by proposing a proton-rich νi process in high-entropy neutrino-driven winds from hypernovae as a viable lanthanide source. It uses two representative wind trajectories and enhanced neutrino fluxes to model nucleosynthesis with the PRISM network, showing robust production up to $A\sim200$ and identifying observational signatures. The results indicate that νi-processed material can reproduce lanthanide-rich patterns in some CEMP-$r$ and CEMP-$r$/$s$ stars and can modestly impact Galactic europium evolution when included in GCE models, while predicting potential but challenging late-time, lanthanide-driven light-curve features. The study highlights the sensitivity to neutrino physics and nuclear reaction rates, underscoring the need for improved constraints and targeted observations to test the νi-process contribution to galactic chemical evolution.

Abstract

The astrophysical origin of the lanthanides is an open question in nuclear astrophysics. Besides the widely studied $s$, $i$, and $r$ processes in moderately-to-strongly neutron-rich environments, an intriguing alternative site for lanthanide production could in fact be robustly $\textit{proton-rich}$ matter outflows from core-collapse supernovae under specific conditions -- in particular, high-entropy winds with enhanced neutrino luminosity and fast dynamical timescales. In this environment, excess protons present after charged particle reactions have ceased can continue to be converted to neutrons by (anti-)neutrino interactions, producing a neutron capture reaction flow up to A~200. This scenario, christened the $νi$ process in a recent paper, has previously been discussed as a possibility. Here, we examine the prospects for $νi$ process through the lens of stellar abundance patterns, bolometric lightcurves, and galactic chemical evolution models, with a particular focus on hypernovae as candidate sites. We identify specific lanthanide signatures for which the $νi$ process can provide a credible alternative to $r$/$i$ processes.

Figures (5)

• Figure 1: Top panel: The final abundance patterns of simulations with the Duan2011 or Wanajo2011-s150 matter trajectory combined with various symmetric neutrino calculations (cyan and purple dashed lines: for neutrinos with average energy 9 MeV and an increased flux by a factor of 5; blue and pink solid lines: for neutrinos with average energy 13 MeV and an increased flux by a factor of 3; red and green dotted lines: many-body neutrino oscillations calculations from Balantekin2024, plotted as functions of the atomic mass number $A$. Bottom panel: Abundances in the $N$-$Z$ plane at the time when the nucleosynthesis pathway shifts neutron-rich and at its maximum extent, corresponding to a temperature $T\sim0.3\ $GK.
• Figure 2: Abundance pattern of the CEMP-$r$ star J2036-0714. The black circles represent the observed abundances, while the solid green line represents the $\nu i$-process abundance pattern from baseline calculation Wanajo150. The orange and blue dashed lines denote the solar $r$-process and AGB $s$-process abundance patterns (1.3 $M_\odot$, $[\mathrm{Fe/H}]=-2.6$, ST/150), respectively. The $\chi^{2}$ values displayed in the legend are calculated using elements with $Z \geq 56$. The residuals between the observed stellar abundances and the theoretical model are presented in the sub-panel beneath the figure.
• Figure 3: Abundance patterns of CEMP-$r$/$s$ stars 97508 (top panel), HE 2208-1239 (bottom left panel), and HE 0243-3044 (bottom right panel). The black points with error bars represent the observed abundances, while the red and blue solid lines correspond to the $\nu i$-process combined with the AGB $s$-process and the solar $r$-process combined with AGB $s$-process abundance patterns, respectively. Among the two $\nu i$ process models that we consider, the best-fit for 97508 and HE 2208-1239 was found to be the Duan2011 model, and the best-fit for the HE 0243-3044 pattern was the Wanajo150 model. The adopted AGB $s$-process models (1.5 M$_\odot$, $[\mathrm{Fe/H}]$=-1.6, ST for 97508; 1.5 M$_\odot$, $[\mathrm{Fe/H}]$=-2.6, ST/3 for HE 2208-1239; 1.5 M$_\odot$, $[\mathrm{Fe/H}]$=-2.6, ST/2 for HE 0243-3044) represent the best-fit solutions in the solar $r$-process plus AGB $s$-process scenario. For comparison, the yellow dashed line shows the predicted abundance pattern from the $i$-process nucleosynthesis 2024A_A...684A.206C. The $\chi^{2}$ values provided in the legend are calculated for elements with Z $\geq$ 38. The average fractional contribution of each nucleosynthetic process is indicated in parentheses following its label. The sub-panels beneath each figure show the corresponding residuals.
• Figure 4: $[\mathrm{Eu/Fe}]$ as a function of $[\mathrm{Fe/H}]$. The plot displays predictions for models incorporating a fiducial NSM contribution, with additional yields from the $\nu i$ process, treated as a rare type of core-collapse supernova (CCSN). We explore different Delay Time Distribution (DTD) functions for NSMs: two purple lines represent models using a constant 100 Myr DTD; blue lines represent models with a $t^{-1}$ DTD; and red lines represent models with a $t^{-2}$ DTD. Dashed lines represent trendlines obtained via inclusion of a $\nu i$ process. For the $t^{-1}$ DTD models, we further investigate the impact of varying the $\nu i$-process rate (from 10% to 1% of the normal CCSNe rate) on lanthanide enrichment (compared via dotted, dot-dashed, and dashed lines). Observational data points for $[\mathrm{Eu/Fe}]$ in Milky Way stars are from the database compiled by NuPyCEE's STELLAB module cote2017jina, which includes data from 2009Roederer_obs2015Jacobson2017Hansen2014Roederer2004Venn2016Battistini. The black, dashed horizontal and vertical lines represent the time corresponding to the formation of the Solar System when $[\mathrm{Fe/H}] = [\mathrm{Eu/Fe}] = 0$.
• Figure 5: Bolometric lightcurves of the $\nu i$-process (for the Wanajo150 model) in a stripped-envelop Type Ic supernova neutrino driven wind under 'compact' (left) and 'dilution' (right) scenarios. Both scenarios have $M_{ej}$ = 1.6$M_\odot$ , $\beta_{ej}$ = 0.04, $M_{^{56}Ni}$ = 0.073 $M_\odot$, and $M_{\nu i}=3\times10^{-4}M_\odot$. The mass of the $\nu i$-process (lanthanides) neutrino-driven wind in the core is 0.03 $M_\odot$ (left) and 0.896 $M_\odot$ (right), corresponding to $\Psi_{mix}=0.01875$ and 0.56, and $f_{lan}$ = 0.01 and 0.00034 respectively, with higher $\Psi_{mix}$ favoring $L_{neb}^{lan}$ over $L_{neb}^{sn}$. The vertical, dotted grey lines indicate $t = t_{tr}$, the time at which the outer lanthanide-free layer becomes transparent. The evolution of $L_{ph}$ slows at this point in response to the higher opacity of the core. For details of the calculation, see the text in section \ref{['sec:lightcurve']}.