$\star$ Dwarf white + giant red

The initial system is typically made up of a star of 7 solar masses and of a star of 2 solar masses (stage A of Figure 5.9 ).

When the mass of the helium heart of the first reaches a value ranging between 1 and 1.4 $\rm \, M_\odot$, it fills its lobe with Rock (B). This involves the formation of an envelope common to both stars (C). After this common period the system is relatively tight (between 4 and 30 $\rm \, R_\odot$, stage D). Then, the principal star contracts and lights its helium. It remains on the principal sequence `` helium '' during ten million years. Progressively with the combustion of helium, there is formation of a heart Co. When this one reached the solar mass of 1 mass, the star becomes a red giantess, fills up once again her lobe of Rock and starts to transfer from the matter rich in helium to his/her companion (E). The second star receives thus from 0.1 to 0.4 $\rm \, M_\odot$ and becomes a star rich in helium (F) while the first becomes dwarf white a Co.

Figure 5.9: Evolution towards a supernova of the Ia type of a binary system composed of a star on 7 $\rm \, M_\odot$ drawn from hachisu2001a

The second star continues its evolution, fills in its turn its lobe of Rock and starts to transmit the matter rich in helium inherited his/her companion (G). The stellar wind intervenes again to stabilize the mass transfer (H). The dwarf white one then increases its mass to reach the mass of Chandrasekhar and explodes (J).

The recent discovery of binary X-soft makes it possible to have a realistic candidate for this kind of star progenitor. They are interpreted like the dwarf ones accrétant of the matter of a companion producing a static fusion on their surface. In theory, these objects can accréter several tenth of solar mass with an accretion rate of $10^{-7 } \rm \, M_\odot añ{-1}$. The life time of these objects is estimated at a billion years, which corresponds to the progenitor well that we seek. Their number seems enough large to be able to account for the rates of explosion of supernovæ of the type Ia. Cependant, the existence of binary X very quickly variable (of about a week) seems to be in dissension with a static fusion on the surface of star, they remain at the present time the best candidates for the simply degenerated systems.