The development of ultra-massive carbon-oxygen white dwarfs


White dwarf (WD) stars are the most numerous members of the stellar graveyard. It is widely recognized that more than 97% of the stars in the universe will turn into WD. These many objects would be a vital tool for understanding the development and evolution of stars and the history of stellar populations and the galaxy.

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In an article published in Royal Astronomical Society Monthly NoticesA research team led by Assistant Professor WU Chengyuan of the Yunnan Observatories of the Chinese Academy of Sciences explored the formation of ultra-massive carbon-oxygen white dwarfs (UMCOWD).

Based on stellar evolution models, WDs with masses less than about 0.45 M⊙ are helium (He) WDs and those with masses between 0.45 and 1.05 M⊙ are carbon-oxygen (CO) WDs. WDs possessing masses greater than 1.05 M⊙ could harbor oxygen-neon (ONe) nuclei and are generally referred to as ultra-massive WDs (UMWDs).

UMWDs play a key role in our understanding of Type Ia supernova explosions, the occurrence of physical processes in the asymptotic phase of giant branches, the existence of high-field magnetic WDs, and the occurrence of double mergers. WD.

Chengyuan Wu, Assistant Professor, Yunnan Observatories, Chinese Academy of Sciences

Recently, Gaia data revealed a development in UMWDs on the Hertzsprung-Russell diagram, which specifies that an additional cooling delay mechanism such as elemental crystallization and sedimentation could be present in UMWDs.

Additional studies have proposed that some UMWDs should have experienced quite long cooling delays, suggesting that they are CO WDs. However, the mechanism of development of these UMCOWDs remains unclear.

In this research, scientists investigated whether the fusion of huge CO WDs with He WDs could develop into UMCOWD. The results of 3D dynamic simulations on the double WD fusion reveal that the double WD fusion is a very fast process which can form a hot corona on the primary WD.

In order to construct the initial structures of the fusion remnants, we adopted the rapid accretion method to simulate the fusion process in the 1D models, and obtained the remnant structures similar to those in the 3D models.

Chengyuan Wu, Assistant Professor, Yunnan Observatories, Chinese Academy of Sciences

After constructing the structures of the remnants of the merger, the scientists found that the post-merger evolution of the remnants is analogous to R Coronae Borealis (R CrB) stars. Helium combustion of the He shell results in the mass expansion of the CO nucleus.

The final mass of CO WD is impacted by the rate of wind mass loss during post-melt evolution, and cannot exceed about 1.2 M⊙. Remains with core masses greater than 1.2 M⊙ will experience surface carbon ignition, which could end their life as ONe WD.

The present results suggest that at least some UMWDs that experience extra-long cooling delays may stem from the unification of CO WDs and He WDs.

Journal reference:

Wu, C. et al. (2022) Formation of ultra-massive carbon-oxygen white dwarfs from the fusion of pairs of carbon-oxygen and helium white dwarfs. Royal Astronomical Society Monthly Notices.



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