Forging neon-distilling white dwarfs in the stellar engulfments of helium white dwarfs

TL;DR

The paper demonstrates that mergers between carbon–oxygen and helium white dwarfs (MS+HeWD and RG+HeWD) can yield CO white dwarfs with ${}^{22}\mathrm{Ne}$ abundances $X(^{22}\mathrm{Ne}) \gtrsim 3\%$, enabling immediate ${}^{22}\mathrm{Ne}$ distillation upon crystallization. This results from off-center, energetic helium flashes that dredge up ${}^{12}\mathrm{C}$, which is converted to ${}^{22}\mathrm{Ne}$ via hydrogen and helium burning, producing final COWD masses around $0.66$–$0.73\,M_\odot$ and Ne mass fractions of roughly $3$–$4\%$. Thermohaline mixing efficiently homogenizes the interior so distillation proceeds promptly as cooling begins; VLTPs can alter surface hydrogen and the spectral type (DA/DB/DAHe), with magnetic fields potentially preserving hydrogen atmospheres and supporting DAHe phenomena. While these channels plausibly account for some DAHe WDs and related cooling delays, they do not explain the ultramassive Q-branch WDs; uncertainties in merger dynamics, hydrogen retention, rotation, and convective overshoot remain, warranting further parameter studies and rate estimates.

Abstract

Once carbon--oxygen white dwarfs cool sufficiently, they crystallize from the inside out. If the white dwarf is rich enough in ${}^{22}\mathrm{Ne}$, these crystallized solids are buoyant and rapidly rise, efficiently liberating potential energy which may halt the cooling of the white dwarf or power magnetic phenomena. Although this ${}^{22}\mathrm{Ne}$ distillation process may explain the cooling anomaly in Q-branch white dwarfs and anomalous emission lines in DAHe white dwarfs, its operation demands unusually high ${}^{22}\mathrm{Ne}$ abundances not generically predicted by isolated stellar evolution. We show that the engulfments of helium white dwarfs by both main-sequence and red giant stars can result in carbon--oxygen white dwarfs with ${}^{22}\mathrm{Ne}$ abundances high enough to distill ${}^{22}\mathrm{Ne}$. This enhancement occurs because carbon dredged up following an especially energetic and off-center helium flash can be processed into ${}^{22}\mathrm{Ne}$ by subsequent hydrogen shell burning and helium shell burning. ${}^{22}\mathrm{Ne}$-distilling white dwarfs from these merger channels are predicted to be somewhat more massive than typical white dwarfs (up to $\simeq0.7M_\odot$) and may have anomalous rotation rates, consistent with DAHe white dwarfs. These binary formation channels for ${}^{22}\mathrm{Ne}$-rich white dwarfs reveal new connections between binary interactions and white dwarf cooling phenomena.

Figures (5)

• Figure 1: A cartoon outlining the MS+HeWD and RG+HeWD merger channels for forming COWDs which are capable of ${}^{22}\mathrm{Ne}$ distillation. In both cases, a mixing event following a highly off-center helium flash dredges up a substantial amount of ${}^{12}\mathrm{C}$, which can later be processed into ${}^{22}\mathrm{Ne}$.
• Figure 2: Temperature profiles for two representative $M=0.80M_\odot$ merger remnants: a MS+HeWD merger remnant with $M_{\mathrm{HeWD}}=0.40M_\odot$ and $M_{\mathrm{env}}=0.40M_\odot$ (dashed) and a RG+HeWD (somewhat non-conservative) merger remnant with $M_{\mathrm{HeWD}}=0.40M_\odot$, $M_{\mathrm{RGcore}}=0.20M_\odot$, and $M_{\mathrm{env}}=0.20M_\odot$ (dash-dotted). Profiles are shown soon after the merger (first column from left), immediately before degenerate helium ignition (second), at the onset of core helium burning (third), and once core helium has been exhausted (fourth). The red and blue portions of the curves denote the hydrogen-rich envelope and helium core, with the black circle denoting the boundary between them (defined by $X=10^{-3}$). The dotted orange line indicates the approximate temperature for helium burning ($T\approx10^8\,\mathrm{K}$). The inner mass coordinates of a $1.2M_\odot$ isolated RG model (starting from $M_{\mathrm{core}}=0.40M_\odot$) are shown for comparison (translucent solid).
• Figure 3: Composition profiles of an isolated RG, fiducial MS+HeWD merger remnant (Section \ref{['sect:making_mshewd']}), and fiducial RG+HeWD merger remnant (Section \ref{['sect:making_rghewd']}) after expulsion of their hydrogen-rich envelopes at the beginning of their COWD cooling tracks ($\log g=6.0$).
• Figure 4: Final masses $M_{\mathrm{COWD}}$ and total ${}^{22}\mathrm{Ne}$ mass fractions $X({}^{22}\mathrm{Ne})_{\mathrm{tot}}$ of the COWDs formed by our merger remnant models.
• Figure 5: The mass fraction profile of ${}^{22}\mathrm{Ne}$ in the COWD resulting from the fiducial RG+HeWD merger remnant. The profile is shown both for the beginning of the COWD phase and at the onset of CO crystallization, by which time thermohaline mixing has fully homogenized its internal composition.