Post-merger evolution of double helium white dwarfs and distribution of helium-rich hot subdwarfs

The mergers of double helium white dwarfs are believed to form isolated helium-rich hot subdwarfs. Observations show that helium-rich hot subdwarfs can be divided into two subgroups based on whether the surface is carbon-rich or carbon-normal. However, it is not clear whether this distribution directly comes from binary evolution. We adopt the binary population synthesis to obtain the population of single helium-rich hot subdwarfs according to the merger channel of double helium white dwarfs. We find that the merger channel can represent the two subgroups in the T eff −log g plane related to different masses of progenitor helium white dwarfs. For Z  = 0.02, the birth rate and local density of helium-rich hot subdwarf stars from the merger of two helium white dwarfs are $\sim 4.82 \times 10^{-3}\, \rm yr^{-1}$ and ∼ 290.0 $\rm kpc^{-3}$ at 13.7 Gyr in our Galaxy, respectively. The proportions of carbon-rich and carbon-normal helium-rich hot subdwarfs are 32 per cent and 68 per cent, respectively.