Faint supernovae and hyper-runaway white-dwarfs from single He-detonation in double HeCO-white-dwarf mergers

TL;DR

This paper investigates mergers of two low-mass hybrid HeCO white dwarfs using 3D hydrodynamics with AREPO and an extended nuclear network, complemented by 1D MESA evolution for the remnant. The key finding is that helium-shell detonations in these systems are incomplete and do not ignite the CO core, producing a faint, fast transient with about $0.13\,M_\odot$ of ejecta and negligible $^{56}$Ni, while the remnant remains bound and acquires a recoil of ~$370\ \mathrm{km\,s^{-1}}$. Long-term evolution suggests the remnant becomes a hot, rotating, PG1159-like CO WD after $\sim$1 Gyr, with surface abundances shaped by mixing of He, intermediate elements, and iron-group isotopes. These results expand the diversity of thermonuclear transients from WD mergers and provide a potential observational link to high-velocity PG1159 stars, with prospects for detection by LSST and related surveys.

Abstract

We present three-dimensional hydrodynamical simulations of mergers between low-mass hybrid HeCO white dwarfs (WDs), offering new insights into the diversity of thermonuclear transients. Unlike previously studied mergers involving higher-mass HeCO WDs and CO WDs, where helium detonation often triggers core ignition, our simulations reveal incomplete helium shell detonations in comparable-mass, lower-mass WD pairs. The result is a faint, rapidly evolving transient driven by the ejection of intermediate-mass elements and radioactive isotopes such as $^{48}$Cr and $^{52}$Fe, without significant $^{56}$Ni production. These transients may be detectable in upcoming wide-field surveys and could account for a subset of faint thermonuclear supernovae. Long-term evolution of the merger remnant shows that high-velocity PG-1159-type stars might be formed through this scenario, similar to normal CO-CO white dwarf mergers. This work expands our understanding of white dwarf mergers and their implications for nucleosynthesis and stellar evolution.

Figures (4)

• Figure 1: The panels show the time evolution from the time of the secondary disruption (left panels), to ignition of the helium (second from left), to the time when the shock converges (middle panels), and finally the ejected remnant in the right panels, still rotating and mixing the bound elements.
• Figure 2: Production of radioactive elements throughout the merger. The incomplete burning of the primary led to the formation of a very low amount of $^{56}$Ni, and more production of lighter elements such as the fast-decaying radioactive elements $^{48}$Cr and $^{52}$Fe that will still produce a luminous transient.
• Figure 3: upper panel: Energy scales of the full model. bottom panel: Structural profile in $\log\rho-T$ space for the Arepo model and the initial MESA white dwarf showing the difference in the thermal state.
• Figure 4: Post-merger evolutionary track in HR and Kiel diagrams. PG1159-type stars are plotted in the Kiel diagram. The black arrow in the HR diagram represents $10^5$ yr. The two hottest stars in the kiel diagram are H1504+65 and RX J0439.8-6809.