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

Abstract

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

23 pages, 9 figures, 6 tables, accepted for publication in MNRAS

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Cited by in corpus (6)

• Structure and evolution of ultra-massive white dwarfs in general relativity
• Can we reveal the core-chemical composition of ultra-massive white dwarfs through their magnetic fields?
• Carbon-oxygen ultra-massive white dwarfs in general relativity
• Minute-Cadence Observations of the LAMOST Fields with the TMTS: II. Catalogues of Short-Period Variable Stars from the First Two-Year Surveys
• Revealing Double White Dwarf Mergers with Multi-messenger Signals
• The evolution of relative frequencies of ONe and CO SNe Ia