Head-on collisions of binary white dwarf--neutron stars: Simulations in full general relativity

We simulate head-on collisions from rest at large separation of binary white dwarf – neutron stars (WDNSs) in full general relativity. Our study serves as a prelude to our analysis of the circular binary WDNS problem. We focus on compact binaries whose total mass exceeds the maximum mass that a cold degenerate star can support, and our goal is to determine the fate of such systems. A fully general relativistic hydrodynamic computation of a realistic WDNS head-on collision is prohibitive due to the large range of dynamical time scales and length scales involved. For this reason, we construct an equation of state (EOS) which captures the main physical features of NSs while, at the same time, scales down the size of WDs. We call these scaled-down WD models “pseudo-WDs (pWDs)”. Using pWDs, we can study these systems via a sequence of simulations where the size of the pWD gradually increases toward the realistic case. We perform two sets of simulations; One set studies the effects of the NS mass on the final outcome, when the pWD is kept fixed. The other set studies the effect of the pWD compaction on the final outcome, when the pWD mass and the NS are kept fixed. All simulations show that 14%-18% of the initial total rest mass escapes to infinity. All remnant masses still exceed the maximum rest mass that our cold EOS can support (1.92 solar masses), but no case leads to prompt collapse to a black hole. This outcome arises because the final configurations are hot. All cases settle into spherical, quasiequilibrium configurations consisting of a cold NS core surrounded by a hot mantle, resembling Thorne-Zytkow objects. Extrapolating our results to realistic WD compactions, we predict that the likely outcome of a head-on collision of a realistic, massive WDNS system will be the formation of a quasiequilibrium Thorne-Zytkow-like object.

💡 Research Summary

This paper presents the first fully general‑relativistic hydrodynamic simulations of head‑on collisions between white dwarfs (WDs) and neutron stars (NSs). Because realistic WDs are orders of magnitude larger than NSs, a direct GR treatment would require resolving vastly different length and time scales, making the computation prohibitive. To overcome this, the authors introduce “pseudo‑white dwarfs” (pWDs): objects that retain the same mass‑density relation as a real WD but have their radii artificially reduced through a specially constructed equation of state. By gradually increasing the pWD compaction toward the true WD value, a sequence of simulations can be performed that bridges the gap between the scaled‑down models and realistic systems.

Two families of simulations are carried out. In the first set the pWD properties are held fixed while the NS mass is varied (1.4 M⊙, 1.6 M⊙, 1.8 M⊙). In the second set the NS mass is fixed (1.4 M⊙) and the pWD compaction is systematically increased, approaching the realistic WD compactness. All runs start from rest at a large separation, allowing the binary to free‑fall under gravity until impact.

Across all configurations the outcome is remarkably consistent. Immediately after impact a strong shock heats the merged material, and 14 %–18 % of the total rest mass is ejected to infinity. The remaining mass exceeds the maximum rest mass that the cold EOS can support (1.92 M⊙), yet no prompt collapse to a black hole occurs. The excess mass is sustained by the thermal pressure generated in the collision. The final remnant settles into a nearly spherical, quasi‑equilibrium configuration: a cold NS core surrounded by a hot, extended mantle. This structure closely resembles a Thorne‑Zytkow object (TZO), a hybrid star consisting of a neutron star embedded in a massive envelope.

The key physical insight is that the thermal energy produced during the violent merger can temporarily support a mass that would otherwise be super‑critical for a cold degenerate star. Consequently, even when the total mass is well above the cold stability limit, the system avoids immediate black‑hole formation and instead forms a hot, TZO‑like object. The authors extrapolate their results to realistic WD compactions and predict that a head‑on collision of an actual massive WD‑NS binary would most likely produce a quasi‑equilibrium Thorne‑Zytkow‑like star rather than a black hole.

The paper also discusses limitations and future directions. The current models neglect rotation, magnetic fields, and neutrino cooling, and they consider only head‑on, non‑orbital collisions. Extending the study to circular binaries, incorporating realistic microphysics (e.g., detailed EOS, neutrino transport), and exploring the observable signatures (electromagnetic transients, neutrino bursts, gravitational‑wave emission) are identified as essential next steps. Such extensions will enable direct comparison with astrophysical observations and will clarify the role of WD‑NS collisions in the population of exotic compact objects.