$\star$ Dwarf white + star on the principal sequence

More attentively let us examine the evolution of a system simply degenerated composed of dwarf white and a red giantess (Figure 5.8 ). The initial system is composed of two stars on the principal sequence (stage A). The first star evolves/moves as a red giantess and evacuates her external layers by stellar wind (B). If this stellar wind is slower than the speed of orbit, the wind carries the angular momentum and the two stars approach to form a tight binary system (this bringing together can reduce the distance between two stars of a factor 10). The two stars are based then in a common envelope (C), then the red giantess evolves/moves into dwarf white carbon-oxygen (D).

Figure 5.8: Evolution towards a supernova of the Ia type of a binary system composed of a star from 6 to 9 $\rm \, M_\odot$ (drawn from hachisu2001a).

The second star continues its evolution on the principal sequence (D). When this one evolves in its turn to the stage of red giantess, it fills its lobe with Rock (E) and the transfer mass towards his/her companion starts. The dwarf white one evacuates part of the mass received by stellar wind and thus stabilizes the transfer (F). It can thus continuously increase its mass to reach the mass of Chandrasekhar and to explode (G and H).