Exploring the formation of extremely massive white dwarfs made of carbon and oxygen

white dwarf star

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White dwarf stars (WDs) are the most numerous members of stellar cemeteries. It is widely accepted that more than 97% of stars in the universe will evolve into WDs. These many objects are a powerful tool for understanding star formation and evolution, and the history of our galaxies and star clusters.

In a study published in Monthly Notices of the Royal Astronomical Societya research group led by Associate Professor Wu Qingyuan of the Yunnan Observatories of the Chinese Academy of Sciences investigated the formation of supermassive carbon and oxygen. white dwarfs (UMCOWDs).

According to stellar evolution models, WDs with masses less than about 0.45 m3 are helium (He) discs, and those with masses between 0.45 and 1.05 m3 are carbon-oxygen (CO) discs. WDs with masses greater than 1.05 M⊙ can contain both oxygen and neon (ONe) nuclei and are usually called large-volume WDs (UMWDs).

“UMWDs play a key role in our understanding of Type Ia supernova explosions, the occurrence of asymptotic giant branch phase physical processes, the existence of high-field magnetic WDs, and the occurrence of binary WD mergers,” Wu said.

Recently Gaia data revealed the improvement of UMWDs on the Hertzsprung-Russell diagram, suggesting the possibility of an additional cooling delay mechanism such as crystallization and initial deposition in UMWDs. Other studies have suggested that some UMWDs must have experienced fairly long cooling delays, which means they are CO2 hard drives. However, the mechanism of formation of these UMCOWDs remains unclear.

In this study, the researchers investigated whether fusions of massive CO2 discs with He WDs can evolve into UMCOWDs. The results of the 3D dynamic simulation of the dual mergers show that the dual WD merger It is a very fast process that can form a hot wreath on a WD primary. “In order to construct the initial structures of the fusion residue, we adopted the rapid accumulation method to simulate the fusion process in 1D models, and obtained the remaining structures similar to those in the 3D models,” Wu said.

After constructing the structures of the fusion residues, the researchers found that the evolution of the remains after fusion is similar to that of R Coronae Borealis (R CrB) stars. The combustion of the helium crust causes the mass of the carbon dioxide nucleus to grow. The final mass of the CO WD is affected by the rate of wind mass loss during post-fusion evolution, and it cannot exceed about 1.2 m3. Residues with core masses greater than 1.2 m3 will be ignited by surface carbon, which may finally end their life as ONe WD discs.

The present results suggest that at least some UMWD devices that experience very long cooling delays may result from the combination of CO WD and He WD devices.

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more information:
Chengyuan Wu et al, Formation of extremely massive carbon-oxygen white dwarfs from fusion of carbon-oxygen pairs and helium white dwarfs, Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac273

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