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Ultra-massive white dwarfs (UMWDs) with masses larger than 1.05Msun arebasically believed to harbour oxygen-neon (ONe) cores. Recently, Gaia datareveals an enhancement of UMWDs on Hertzsprung-Russell diagram (HRD), whichindicates that extra cooling delay mechanism such as crystallization andelemental sedimentation may exist in the UMWDs. Further studies suggested thata portion of UMWDs should have experienced pretty long cooling delays, implyingthat they are carbon-oxygen (CO) WDs. However, the formation mechanism of theseUMCOWDs is still under debate. In this work, we investigated whether the mergesof massive CO WDs with helium WDs (He WDs) can evolve to UMCOWDs. By employingstellar evolution code MESA, we construct double WD merger remnants toinvestigate their final fates. We found that the post-merger evolution of theremnants are similar to R CrB stars. The helium burning of the He shell leadsto the mass growing of the CO core at a rate from 2.0*10^-6 to 5.0*10^-6Msun/yr . The final CO WD mass is influenced by the wind-mass-loss rate duringthe post-merger evolution, and cannot exceed about 1.2Msun. The remnants withcore mass larger than 1.2Msun will experience surface carbon ignition, whichmay finally end their lives as ONe WDs. Current results implies that at least aportion of UMWDs which experience extra long cooling delay may stem frommerging of CO WDs and He WDs.

Formation of ultra-massive carbon-oxygen white dwarfs from the merger of carbon-oxygen and helium white dwarf pairs