Localized $^{18}$O production in white dwarf mergers

TL;DR

This work tests whether a He WD + CO WD merger can yield RC B–like low $^{16}$O/$^{18}$O ratios within the stars’ lifetimes by performing a 3D MHD merger simulation with Arepo, followed by detailed NuGrid post-processing nucleosynthesis. It reveals that an extended shell-of-fire forms around the CO core, imprinting an asymmetric chemical structure that persists into the long-term evolution. The study identifies the thick SoF and the production channel $^{14}$C(alpha,gamma)$^{18}$O as key to sustaining low $^{16}$O/$^{18}$O in outer zones for timescales of $10^2$–$10^3$ years, particularly when a modest proton fraction is present. Overall, the results show that spatially and temporally varying conditions in the SoF can produce RC B–like $^{16}$O/$^{18}$O ratios, underscoring the importance of multidimensional modeling and long-term evolution in WD merger nucleosynthesis.

Abstract

The merger of a He white dwarf (WD) and a CO WD is the favored formation channel for R Coronae Borealis (RCB) stars. These stars exhibit ${^{16}}\mathrm{O}/{^{18}}\mathrm{O}$ ratios that are orders of magnitude lower than the solar value. However, it is not fully understood whether such low ${^{16}}\mathrm{O}/{^{18}}\mathrm{O}$ ratios can be achieved in WD merger remnants for the predicted lifetime of RCB stars of around $10^4\,\mathrm{years}$. In this work, we perform detailed nucleosynthesis calculations of a 3D magnetohydrodynamical simulation of a merger of a $0.3\,M_\odot$ He WD and a $0.6\,M_\odot$ CO WD for $4000\,\mathrm{s}$ at which point a steady state in temperature and density is reached. From this point, we follow several radial zones to study the long-term production of ${^{18}}\mathrm{O}$ and its variability throughout the burning region. We find that the asymmetric merger process leaves an imprint on the distribution of the abundances at the end of our hydrodynamic simulation. During the long-term evolution up to $100\,\mathrm{years}$, we observe ${^{16}}\mathrm{O}/{^{18}}\mathrm{O}$ ratios of order of unity, although the timescale on which ${^{18}}\mathrm{O}$ is destroyed again is highly location dependent. Importantly, our calculations suggest that in the outer layers of the burning shell, the dominant production channel is $^{14}\mathrm{C}(α,γ)^{18}\mathrm{O}$ instead of the commonly considered $^{14}\mathrm{N}(α,γ)^{18}\mathrm{F}(β^+)^{18}\mathrm{O}$ reaction, whereby the former can be sustained for longer periods of time. Furthermore, these outer regions do not reach the conditions necessary for fast $α$-captures in ${^{18}}\mathrm{O}$ to ${^{22}}\mathrm{Ne}$, thus being favorable to maintaining a low ${^{16}}\mathrm{O}/{^{18}}\mathrm{O}$ ratio.

Figures (4)

• Figure 1: Illustration of the merged HeCO WD at different timesteps. The top and middle row show a planar temperature slice along the rotational and equatorial plane, respectively. Here, it can bee seen that initially strong asymmetries are present which get smoothed out over time. The lower row shows spherically averaged shells centered on the center of mass of the CO core. The orange band highlights the SoF investigated in the single-zone models; the dashed vertical lines indicate the zones discussed in-depth in Section \ref{['sec:single_zone']}.
• Figure 2: Spherically averaged tracer particle yields. Here, the binning is the same as in Figure \ref{['fig:merger']}. The top row shows the temperature and density values. The middle and bottom row illustrate the yields of various isotopes, both for the $X(^1\mathrm{H})=0.0$ and $X(^1\mathrm{H})=0.01$ case, respectively. In all rows, we also indicate the 3D nature of the tracer particles by adding each individual tracer particle value for select quantities. As in Figure;\ref{['fig:merger']}, the dashed vertical lines indicate the zones discussed in Section \ref{['sec:single_zone']}. Note that the abundances for $^{12}\mathrm{C}$ and $^{16}\mathrm{O}$ overlap in this figure.
• Figure 3: Illustration of the results of the single-zone network for three select zones. Top row: Abundance evolution of the zones discussed in Section \ref{['sec:single_zone']}. Here, we show both the $X(^1\mathrm{H})=0.0$ case (solid lines) and the $X(^1\mathrm{H})=0.01$ case (dotted lines). Bottom row: Evolution of the $^{16}\mathrm{O}/^{18}\mathrm{O}$ ratio over time for both $^{1}\mathrm{H}$ fractions. In the individual columns, we also indicate the temperatures and densities present in the respective zones.
• Figure 4: Illustration of the time evolution of the $^{16}\mathrm{O}/^{18}\mathrm{O}$ ratio for each zone in the region highlighted in Figure \ref{['fig:merger']}. The left column shows the $X(^1\mathrm{H}) = 0.0$ case and the right column the $X(^1\mathrm{H}) = 0.01$ case.