What happens after a merger of massive double white dwarfs?

Double white dwarf (WD) binaries are important gravitational wave sources in our galaxy, and their mass is related to the type Ia supernovae, electron capture supernovae and millisecond pulsars.

Double WDs in binary systems can merge together due to the gravitational wave radiation. However, the evolution of postmerger remnants from such a merger remains unclear.

Now, a research group led by assistant professor Wu Chengyuan from the Yunnan Observatories of the Chinese Academy of Sciences investigated the the evolutionary outcomes from the merger of oxygen-neon WD and carbon-oxygen WD.

The study was published in the Astrophysical Journal Letters on Feb. 22.

The researchers constructed corresponding models to investigate the evolution of merger remnants. They found that such merger remnants can evolve to carbon-oxygen giants, and their evolutionary outcomes are related to their total masses.

Under the fixed wind mass-loss prescription, remnants with masses lighter than 1.90M⊙ could evolve to oxygen-neon WDs, whereas the remnants with masses greater than 1.95M⊙ could experience electron capture supernova explosions to become oxygen-neon-iron WDs.

“Our result implies that the super-Chandrasekhar mass remnants originated from the mergers of oxygen-neon WD and carbon-oxygen WD cannot form neutron stars, which challenged the traditional understandings,” said Wu.

Moreover, they used the corresponding models to explain the oxygen-rich object IRAS 00500+6713 (J005311) located in the infrared nebula in Cassiopeia. The spectrum of this object is similar to that of the oxygen-rich Wolf-Rayet stars, and it has relatively high wind mass-loss rate and extremely high wind velocity.

At present, the origin of this object is still unclear. Wu explained the observational features of this object by using their models, and found that this object was originated from the merger of a 1.08M⊙ oxygen-neon WD with a 0.52M⊙ carbon-oxygen WD.