Recent models for the evolution of super-Chandrasekhar mass carbon-oxygen white dwarf merger remnants by Schwab et al.8 match the properties of WS35 remarkably well. Not only does their fiducial model fit the Hertzsprung-Russell diagram position of WS35 (Fig. 3). They also predict an extended episode of extreme mass loss during and shortly after the merger event in an expanded cool supergiant stage which will form a slowly-expanding hydrogen- and helium-free circumstellar nebula with an expansion velocity of about 100 \( \mathrm { k m \, s ^ { - 1 } } \). After the settling of the post-merger prod-uct in a hot and compact stage, its ultraviolet emission ionizes the surrounding nebula. Its current∼ 100 times faster wind sweeps the preceding slow material into a shell, which appears inside the density-bounded H II region, which forms the diffuse halo (see Fig. 1). The expected hydrogen-and helium-free composition of the nebula and the high temperature of its central star (leading to triple ionization of oxygen) suggest that no optical nebular lines form and that the observed mid-infrared emission from the nebula is dominated by the [O IV] 25.89 \( \mu \) m and [Ne V] 14.32 and 24.32 \( \mu \) m lines (cf. ref.22). This naturally explains why the nebula WS35 does neither appear in the IPHAS image nor in our long-slit spectrum.

The merging white dwarf scenario also addresses the major puzzle about WS35, namely the extreme width of its emission lines. A velocity of 16, 000 \( | \, \mathrm { k m \, s ^ { - 1 } } \) exceeds the stellar escape speed about 10 times, and is typical for supernovae but so far unheard of for radiation driven winds. Pure radiation driving is in fact excluded, since the wind’s kinetic energy flux exceeds the total radiative luminosity of the star by a factor of two (Methods). The extreme velocity can, however, be explained in the framework of rotating magnetic wind models. Poe et al.23 find that a rigidly co-rotating magnetic field can enhance the speed and mass outflow rate of radiation driven winds by more than a factor of three, at the cost of the star’s rotational energy. We find that a co-rotation speed of 16, 000 \( \mathrm { k m \, s ^ { - 1 } } \) at the Alfve´n point in WS35, where the inertia force starts to dominate over the magnetic forces24, requires an Alfve´n radius of about 10 stellar radii \( \sim 1 . 5 ~ \mathrm { R } _ { \odot } ) \)), which is achieved with a magnetic field strength of \( \sim 1 0 ^ { 8 } \) G. Since the whole post-merger evolution is expected to last ∼ 20, 000 yr (ref.8), it is plausible that the corresponding magnetic torques have not yet spun down WS35.

The generation of a strong magnetic field is indeed expected in stellar mergers25. 3D magneto-hydrodynamical models of white dwarf mergers find a magnetization of the merger product of \( 2 \times 1 0 ^ { 8 } \) G (ref.9). This compares to the peak of the B-field distribution of magnetic white dwarfs of several \( 1 0 ^ { 7 } \) G (ref.26). The observations that the mean mass of magnetic white dwarfs is signif-icantly higher than that of non-magnetic ones, and that nearly none of the known magnetic white dwarfs have a companion star provide strong evidence for magnetic field generation by white dwarf merging26.

Adopting the escape velocity from the expanded merger remnant of ∼ 100 \( \mathrm { k m \, s ^ { - 1 } } \)(cf. ref.7) and given the angular radius of the nebula of 1.6 pc, we derive an expansion age of ∼ 16, 000 yr for WS35. Together with the high temperature of the central star, this indicates that WS35 is close to the endpoint of its post-merger evolution. Since WS35 is more luminous than the \( 1 . 4 9 \, \mathbf { M } _ { \odot } \) model of Schwab et al.8, it appears likely that its mass also exceeds the Chandrasekhar limit, with the exciting perspective that it will produce a low-mass neutron star in the near future, accompanied

by a high-energy transient and a fast evolving supernova11. It is also likely that the inner shell of the circumstellar nebula will persist until the explosion and, given the expected low mass of supernova ejecta, will affect the appearance of the supernova remnant ∼ 100 yr after explosion.

The merging of two stars in a binary system is not a rare event. About 10 per cent of the massive main sequence stars are thought to be merger products27, as well as a similar fraction of the known white dwarfs28. The very unusual wind of WS35 strongly supports the idea that stellar mergers can indeed produce highly magnetized stars, which would explain the magnetic stars of the upper main sequence29 and the formation of the magnetic white dwarfs26. Our results also enlighten the ongoing debate on whether a super-Chandrasekhar mass merger of two carbon-oxygen white dwarfs leads to a Type Ia supernova. Here, WS35 appears to provide a counter example, which will likely produce a neutrino-flash and a gamma-ray burst30, followed by a very fast and subluminous Type Ic supernova11.

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