The systems progéniteurs

We now will interest we in the system progenitor. If it seems from now on established that the supernovæ of the Ia type are thermonuclear explosions of dwarf white, there is no yet clear consensus on the nature of the companion who yields matter to him (Figure 5.7 ). Moreover, the majority of the attempts to observe a system which could give place to this type of explosion are revealed nonconclusive.

Figure 5.7: Binary system in which dwarf white a accréte of the matter of his/her stellar companion. The dwarf white one is in the center of the accretion disc.

No progenitor of supernova of the Ia type was, until now, observed directly. The star progénitrice is indeed of weak luminosity. The systems progéniteurs are thus constrained only by indirect arguments (photometry and spectroscopy). One thus proceeds by elimination by hoping that there remains only one candidate at the end. Unfortunately in the case of the Ia types, no candidate passes for the moment all the tests without ambiguities.

The absence of hydrogen in the spectra implies that the progenitor does not contain with any more but $0.1 \rm \, M_\odot$ hydrogen. Moreover, the very significant presence of the silicon line with 6355 angstroems indicates the production of elements of intermediate mass by fusion. Speeds of expansion of the layers of $1 \rm \, M_\odot$ amalgamating out of iron, silicon and calcium. A fast calculation of the kinetic energy produced during the explosion of a star of a solar mass gives:

$$\frac{1}{2} m v^2 = 1 \rm\, M_\odot \times (10\,000 \rm\, km\,s^{-1})^2 = 1\times 10^{44} J = 1 \times 10^{51} ergs$$

that is to say the energy produced during the explosion of a supernova of the Ia type.

The emission in ultraviolet ray and its exponential decay indicate a progenitor of less $10\, 000\rm \, km$ of ray. Indeed, to make it possible to reach speeds of ejection observed, it is necessary that the layers external of star are sufficiently thin allowing a fast evacuation of the energy photons.

After approximately two weeks, the spectrum which was dominated by intermediate elements of masses exhibe mainly of iron ionized once (FeII) as well as cobalt ionized twice (CoIII), which again indicates the thermonuclear explosion of a compact object. This implies that the stars at the origin of these explosions are stars from 6 to 8 solar masses during their life on the principal sequence of diagram HR.

Lastly, nearly 85% of the supernovæ of the Ia type have luminosities to the maximum, lightcurves and spectra very similar. The dispersion of luminosity to the peak of light is approximately 0.4 magnitude (40%). This imposes a relatively homogeneous class of progéniteurs.

However, the observation of supernovæ of very weak luminosity like SN1991bg with a luminosity to the maximum nearly 3 magnitudes less brilliant than these congeneric, and to 1991T, with a higher luminosity to the maximum 0.5 magnitude, show that it is difficult to find a class single progéniteurs to give an account of all the observations.

The observation of Ia so much in the spiral galaxies than in the elliptic galaxies excludes massive stars if a single progenitor is considered. On the other hand, the supernovæ very luminous and evolving/moving slowly (with the manner of 1991T) miss elliptic and irregular galaxies. It seems well that there are several types of progéniteurs.

The observations are overall in agreement with the model of Hoyle & exploding carbon-oxygen Fowler dwarf white near to the mass of Chandrasekhar. Diversity could be due to the history and the nature of dwarf white though the existence of other types of progenitor does not seem completely isolated (dwarf white Oxygen-Neon-Magnesium for example for 1991bg).