III The spectra of IA SNe (Petschek)

III.1 The main spectra feature of a IA SN

In every aspect of astronomical research spectra of the object you are looking at, can tell you an awful lot about that perticular object. The same goes for research in SNe. To find out what processes are taking part during and after an explosion, astronomers measure lots and lots of spectra. But not only tell the spectra you what processes take place. It also tells you something about the objects that are taking part in the process. Take for example the spectrum in fig 1. This shows the spectra of SN 1979C at different times. The first spectrum was taken near the moment where the magnitude was at maximum. Already after one month one perticular line becomes very apparent at about 656 nm (=6560 A). This is the H(alfa) line. The presence of this line indicates that a lot of hydrogen is present. In other words, this can never be the spectrum of a type IA SN, because the progenitor of a type IA SN, a white dwarf (WD) barely contains any hydrogen.
  I'm aware of the fact that I'm turning things around, because I wanted to show the reader proof that this statement is indeed true instead of using this as a fact to proof that this spectrum doesn't belong to a type IA SN, but there is a reason for this. Minkowski was the first in 1940 to abandon the idea that all SNe have essentially the same spectrum and made a division into two main groups: a group with spectra that contains the hydrogen line, which was called Type II and a group that didn't contain that line in the spectra: Type I. Nowadays this main groups have been subdivided into even more groups with a bit different features in the spectra. The result is that today we have Type Ia, Ib and Ic SNe. This further division is basically quite simple. If one takes the spectra of a Type I SN and one finds lines of Si, this SN is referred to as Ia. If there is no Si present, but there is He present, then it is referred to as Ib. If there is no Si and no He present in the spectrum, the SN is put in the Ic-group. However, SNe Ib and Ic are very rare and therefore not further looked at in this text.
  The main point here is the absence of hydrogen lines in the spectrum of a Ia SN. This is shown in figures 2 and 3, which show a typical spectrum of a type Ia SN, in this case SN 1981B. It is very clear that indeed the emission line at 6560 A is 'missing'. Other lines one can see in the spectrum of SN 1981B are from the red to the blue part: Ca II, O I, Si II, S II, Fe II, Ca II. Some 2 weeks after maximum (the second spectrum) one can notice another very important difference. The IA spectrum becomes strongly dominated by lines around 5000 A. These lines are not seen in the Type II spectrum and belong to Fe. This is also a very specific feature of Type IA SNe.

Figure 1: spectra of SN 1979C (Type II)	Figure 2: spectra of SN 1981B (Type IA)
These two figures show the time evolution in the spectra of type Ia and type II SNe. The most important difference can be found at a of about 6560 A. In Type II SNe this line is very prominently present, while in a type I SNe this line is missing. This difference forms the basis for the classification of Type I and II.

III.2 The spectra after four months

A short look at figures 1 and 2 already show that not only the spectrum at maximum looks very different, also the ways in which the spectra change in time are very different. In the SN 1979C (type II) the hydrogen lines become very apparent. This is not the case in SN 1981B. Figure 3 confirmes what seems to happen in figure 2: here the Fe lines become very apparent as are the lines of Co (Co III is the large dip around 6800 A) and ⁵⁶Ni. Remember that this spectrum doesn't belong anymore to a star! This spectrum is from the nebular that is left after the explosion took place.

Figure 3: spectrum of SN 1981B at maximum (same as first spectrum of figure 2, ie the spectrum of March 6-9)	Figure 4: spectrum of the nebula remains of SN 1981B 4 months after maximum
A comparison between the spectra at maximum and the spectra of the nebular phase of a typical SN IA explosion. Typical for a IA SN is the strong Fe lines at the later stage, that soon after the explosion start to appear. A important feature of the late time spectrum is the strength of the 56Ni lines. A lot of explosion models are very sensitive toward the results for this perticular element. Little changes in the parameters of the models tend to create large changes in the total amount of Ni that is created during the explosion.

  So, what can we do with such spectra. Most times these spectra are used to create models for the SN explosion. Parameters that can be adjusted within this models are then adjusted to obtain a spectrum that resembles the observed spectrum as good as possible. A lot of the nowadays models use some form of carbon-detonation or deflagration. Now one nows exactly which of these choices will eventually be the right one. There are also people how are trying to combine these different theories into one theorie: delayed detonation model. One can read all about this models in the section about the physics of type IA SNe. My point here however is, that the basic idea that in type Ia there is a carbon core that explodes is absolutely necessary to account for the measured spectra and there are not many star types that have this property. This gives an new clue that the progenitor must be a WD.
  The lines of ⁵⁶Ni appear to be of extremely great importance for the initial mass of the primary star. The production of this element can change up to 14% if the mass of the primary star that evolves to a WD is changed (Dominguez et all). Therefore precise measurements of this lines can put strong constraints on the initial mass of the primary star.