The spectroscopy

As we saw earlier, the supernovæ of the Ia type present neither hydrogen, nor helium in their spectrum, whereas these elements represent more than 99% of the nuclear matter of the universe. The supernovæ of the Ia type are characterized by a very strong silicon emission ionized once (SiII). It is the emission of this element with 6100 angstroems which is the clearest signature of the types Ia. many other elements of intermediate mass are observed (magnesium, calcium or sulphur) at the time of the maximum of luminosity. They leave room with cobalt and nickel in the following weeks, for finally being dominated by fine-grained iron the phase nébulaire a few months after the explosion. Another significant characteristic of the spectra, is the presence of P-Cygni profiles (Figure 5.1 ) very broad indicating speeds of expansion of the very large external layers (of $50~000\rm \, km\, s^{-1}$).

Figure 5.1: Profile says P-Cygni (bearing the name of the star hypergéante P Cygni) of the lines formed within an atmosphere expanding. This one is seen by the observer partly in projection on photosphere (it also expanding in the case of a supernova) which forms the continuous spectrum -- from where a component in absorption shifted towards blue -- and partly apart from the heart -- what gives a component in emission centered over the wavelength at rest $\lambda_o$. The more the speed of expansion of photosphere is raised, the more the profile is `` dilated '' in wavelength, and the more the minimum of absorption is shifted towards blue, the not moving emission peak. The height of the emission and absorption lines is directly connected to the optical depth: the higher this one is, the more absorption and the emission will be strong until the line becomes saturated if the optical depth is too high. This figure as its legend are extracted from blanc2002.

Lastly, the spectra do not have until now little or not shown polarization, which would tend to show that these phenomena are spherical (howell2001, kasen2003 and wang2003).

Figure 5.2: Spectrum of typical SN1981B of the supernovæ of the Ia type to the maximum of luminosity (drawn from nugent199ä).