  Double white dwarf (WD) merger process and their post-merger evolution are
important in many fields of astronomy, such as supernovae, gamma-ray bursts,
gravitational waves, etc. The evolutionary outcomes of double ultra-massive WD
merger remnants are still a subject of debate, though the general consensus is
that the merger remnant will collapse to form a neutron star. In this work, we
investigate the evolution of a 2.20Msun merger remnant stemmed from the
coalescence of double 1.10Msun ONe WDs. We find that the remnant ignites
off-centre neon burning at the position near the surface of primary WD soon
after the merger, resulting in the stable inwardly propagating oxygen/neon
(O/Ne) flame. The final outcomes of the merger remnant are sensitive to the
effect of convective boundary mixing. If the mixing cannot stall the O/Ne
flame, the flame will reach the centre within 20 years, leading to the
formation of super Chandrasekhar mass silicon core, and its final fate probably
be neutron star (NS) through iron-core-collapse supernova. In contrast, if the
convective mixing is effective enough to prevent the O/Ne flame from reaching
the centre, the merger remnant will undergo electron capture supernova to form
an ONeFe WD. Meanwhile, we find that the wind mass loss process may hardly
alter the final fate of the remnant due to its fast evolution. Our results
imply that the coalescence of double ONe WDs can form short lived giant like
object, but the final outcomes (NS or ONeFe WD) are influenced by the uncertain
convective mixing in O/Ne flame.

Solar and Stellar Astrophysics;High Energy Astrophysical Phenomena