Comparison of Ne-22 core and shell distilled WD detonations in AREPO

TL;DR

This paper uses 3D hydrodynamical simulations with AREPO to study detonations in $1.0\,M_\\odot$ white dwarfs that have undergone $^{22}$Ne distillation during crystallisation, comparing a central-core enriched model ($M_c$), a shell-enriched model ($M_s$), and homogeneous equivalents. The authors quantify how distillation geometry alters nucleosynthetic yields, finding similar $^{56}$Ni production but markedly different iron-group neutron-rich isotopes, with $^{58}$Ni and $^{54}$Fe enhanced in the core-distilled case and $^{55}$Co and $^{15}$N elevated in the shell-distilled scenario; they also show early-time spectra are only modestly different, while nebular-phase signatures promise clearer discriminants. By post-processing with a 384-isotope network and conducting radiative transfer with TARDIS, they connect progenitor crystallisation physics to potential observational fingerprints in Type Ia supernovae, including possible nebular [Ni II] emission and unique abundance stratifications. The results suggest distillation-driven composition gradients can influence SN Ia diversity and offer avenues to probe WD progenitor evolution and cooling histories via late-time spectral diagnostics. Overall, the work highlights how microphysical phase separation in WDs can leave detectable nucleosynthetic fingerprints in a subset of thermonuclear transients, linking Gaia-era WD cooling phenomena to SN Ia observations.

Abstract

We present three-dimensional hydrodynamical simulations of detonations in $1.0 \mathrm{M_{\odot}}$ white dwarfs that have undergone $^{22} \mathrm{Ne}$ distillation during crystallisation. These simulations, conducted with the moving-mesh code AREPO, aim to investigate the effects of chemical separation on the ejecta and spectra of such WDs undergoing thermonuclear explosions. The distillation process alters the internal chemical stratification of the star, concentrating neutron-rich material either in a central core or in an interior shell. We model both configurations as well as a homogeneous equivalent for each case with the same $^{22} \mathrm{Ne}$ content distributed evenly at all radii. Despite similar $^{56} \mathrm{Ni}$ yields between the core and shell models ($0.40$ and $0.45 \mathrm{M_{\odot}}$ respectively), the two models yield markedly different iron-group abundances. Both distilled models showed significantly enhanced production of $^{15} \mathrm{N}$ via the decay of $^{15} \mathrm{O}$. The $^{22} \mathrm{Ne}$-core model produces enhanced amounts of stable neutron-rich iron-group isotopes such as $^{58} \mathrm{Ni}$ and $^{54} \mathrm{Fe}$. We highlight observational signatures associated with these differences, including potentially enhanced [$\mathrm{Ni}_{\rm II}$] lines in nebular spectra. Synthetic TARDIS spectra at early times show only moderate differences. Our results suggest that white dwarf distillation, a process linked to delayed cooling in the Gaia Q branch population, may leave detectable nucleosynthetic fingerprints in a subset of Type Ia supernovae. These findings open additional pathways to probe progenitor evolution and the role of crystallisation in shaping the diversity of thermonuclear transients.

Figures (11)

• Figure 1: Comparison of the initial one-dimensional radial chemical profiles of the core and shell distilled models. Recall that the naming of these two profiles corresponds to the location of the majority of the $^{22} \mathrm{Ne}$ in the structure - i.e. the core distilled model, $M_c$, has a large volume of neon concentrated at the stellar core whereas the shell model, $M_s$, has a thin shell at a larger radius. Note that some density tick labels have been omitted in the atmosphere.
• Figure 2: Number abundance for selected elements of distilled vs homogeneous models 2 Gyr after detonation. For example, a value of 100 per cent indicates that the distilled model produced twice as many atoms of a given element by number.
• Figure 3: Figure partially reproduced from blondinStableNickelProduction2022 - Stable $^{58} \mathrm{Ni}$ yields one pear post explosion versus the radioactive $^{56} \mathrm{Ni}$ yield at $t \approx 0$ for various SN Ia models. The $M_{Ch}$ models are shown in red, while the sub-$M_{Ch}$ models are shown in blue. These include deflagrations, double detonations, and gravitationally confined detonations. Note also that simulations have been divided into 1D, 2D, and 3D. Our new results are shown in black.
• Figure 4: Comparison of elemental composition of the ejecta relative to iron and solar composition 2 Gyr after detonation, i.e. taking into account radioactive decay of unstable isotopes in the ejecta. We use the solar elemental abundances of asplundChemicalCompositionSun2009.
• Figure 5: Figure partially reproduced from kobayashiNewTypeIa2020 - the Ni-Mn diagram constraining SN Ia enrichment. Results are shown for near-$M_{Ch}$ SNe with masses $M_{WD} = 1.38$, $1.37$, $1.33$, and $1.30$. The points indicate initial metallicities of $Z = 0$, $0.002$, $0.01$, $0.02$, $0.04$, $0.06$, and $0.10$ (left to right). Sub-$M_{Ch}$ SNe Ia are also given with masses $M_{WD} = 0.9$, $0.1$, $1.1$, and $1.2$ and metallicities $Z = 0$, $0.001$, $0.002$, $0.004$, $0.01$, $0.02$, and $0.04$ (left to right). The solid lines with filled circles are solar-scaled initial compositions originating from kobayashiNewTypeIa2020, whereas the dashed lines with open squares are $^{22}\mathrm{Ne}$ only yields from leungExplosiveNucleosynthesisNearChandrasekharmass2018leungExplosiveNucleosynthesisSubChandrasekharmass2020. Our new results are shown in black.