The origin of hypervelocity white dwarfs in the merger-disruption of He-CO white dwarfs

TL;DR

This study demonstrates a new formation channel for hypervelocity white dwarfs (HVWDs) via the merger of two low-mass HeCO WDs, where a partial tidal disruption of the secondary and a double detonation of the primary ejecta the remnant core at $\sim2000$ km s$^{-1}$. Using a 3D Arepo simulation, the authors show a bound remnant of $\approx0.492$ M⊙ being launched at $2061$ km s$^{-1}$, while the accompanying SN ejecta are modest, yielding a faint, peculiar transient. Mapping the merger remnant into a 1D MESA evolution reveals a hot, inflated HVWD that cools over tens to hundreds of Myr, with predicted radii and temperatures consistent with several observed HVWDs. Compared to the D6 scenario, this HeCO-merger channel naturally explains both the high ejection velocities and the observed properties of HVWDs, and it implies a broader diversity of thermonuclear transients linked to HeCO WD mergers and related peculiar SNe Ia.

Abstract

Hypervelocity white dwarfs (HVWDs) are stellar remnants moving at speeds exceeding the Milky Way's escape velocity. The origins of the fastest HVWDs are enigmatic, with proposed formation scenarios facing challenges explaining both their extreme velocities and observed properties. Here we report a three-dimensional hydrodynamic simulation of a merger between two hybrid Helium-Carbon-Oxygen white dwarfs (HeCO WDs with masses of 0.69 and 0.62 M$_\odot$). We find that the merger leads to a partial disruption of the secondary WD, coupled with a double-detonation explosion of the primary WD. This launches the remnant core of the secondary WD at a speed of $~2000$ km s$^{-1}$, consistent with observed HVWDs. The low mass of the ejected remnant and its heating from the primary WD's ejecta explain the observed luminosities and temperatures of hot HVWDs, which are otherwise difficult to reconcile with previous models (such as the D6). This discovery establishes a new formation channel for HVWDs and points to a previously unrecognized pathway for producing peculiar Type Ia supernovae and faint explosive transients.

Figures (6)

• Figure 1: WD disruption and shock Propagation. The panels show the time evolution from the time of the ignition of the Helium detonation (left panels) to the time when the shock converges in the CO core of the primary WD (third from the left), and finally the ejected remnant in the right panels. White arrows point to the detonation points, gray arrows indicate the remnant kick velocity.
• Figure 1: WD disruption and shock Propagation. The panels show the time evolution from the time of the partial disruption of the secondary WD (left panels) to the time of the He detonation (second left) when the shock converges in the CO core of the primary WD (third from the left), and finally the ejected remnant in the right panels.
• Figure 2: Production of radioactive elements throughout the merger. The early Helium burning and detonation already produces a significant amount of radioactive elements, with a few 0.01 M$_\odot$ of radioactive $^{56}$Ni, as well as faster decaying radioactive elements $^{48}$Cr and $^{52}$Fe produced at this stage. The bulk of the $^{56}$Ni is produced by the CO detonation beginning at t=878.8 s.
• Figure 2: Evolution of the MESA model and comparison with all observations. Left: The HR diagram for our model compared with the observed values, assuming a hydrogen-dominated atmosphere for J1332 based on ref. Wer+24b. The error bars depict the 1 and 2 $\sigma$ uncertainties. The black arrow marks the point when the star is $10^4$ years old. Right: The radius and temperature evolution of the model as a function of its age. In particular, the HR positions of HVWDs J0546, J1332, and J0927 may be explained by our model, within the 1-sigma (J0546) and 2-sigma (J0927, J1332); as well as the temperatures of these WDs, while the radius of J1332 is inconsistent, if hydrogen atmosphere is assumed for J1332, but see \ref{['sec:SI-J1332']}
• Figure 3: Evolution of the MESA model and comparison with HVWD observations. Left: The HR diagram for our model compared with the observed values. The error bars depict the 1 and 2 $\sigma$ uncertainties. The black arrow marks the point when the star is $10^4$ years old. Right: The radius and temperature evolution of the model as a function of its age. In particular, the HVWDs J0546, and J0927 may be explained by our model, within the 1-sigma (J0546) and $2\sigma$ (J0927; or 1$\sigma$ too, if ref. Elbadry+23 inferred temperature is adapted) uncertainties. See further discussion in the SI regarding J1332 and its omission here 2.4