Supernovae

I.1 Introduction

This part of the project is devoted to the progenitors of type IA supernovae (SNe). Finding this progenitors can help identifying what is exactly going on in a SN of type IA. In practice it works the other way around. In finding processes that can explain the observations of SNe there are found constraints on the progenitors. This basic idea is quite simple off course: if you find that the progenitor must have had a mass of 10M_sun, it is very unlikely that you were looking at a white dwarf that exploded. In the case of type IA there hasn't been much chances in the understanding of the progenitors. In the last say thirty years or so it has been more or less undisputed that the progenitor consists of a binary system of a white dwarf (WD) of a mass a little bit under 1.4M_sun, the socalled Chandrasekhar mass, and a secondary star with a mass of about 0.8 M_sun.

So, why would such a system lead to such a big explosion like SNe, where typically an energy of 10⁵¹ erg is released (Hoyle & Fowler 1960)? And even more, what proof is there, that indeed we have to look at such binary systems? I'll try to answer this questions in several ways. The results I'll show, are generally not very recent because recent research deals a lot more with the details of the mechanisms that cause the explosion it self, all based on the general picture I described above. Information about this kind of research can be found in the section about the processes in type IA SNe. But let me first describe this system of a WD and a secondary star in a bit more detail.

I.2 The generally accepted picture: a binary system consisting of a WD and a secondary companion (Whelan & Iben 1973)

The life of a presupernova system begins as a binary system with a primary star (P) with a mass of about 1.8M_sun ~< M_P0 ~< 3M_sun and a secondary star (S) with a mass of about ~<0.8M_sun. Within a time of tp ~ 3.4*10⁷ yr the primary burns first his hydrogen and then helium in his core. After this, it descends the red-giant branch and forms a carbon-oxygen core. During this evolution of the primary, the secondary remains on the main sequence, because of his lighter mass. During this red-giant fase of the primary, mass-transfer from the primary to the secondary takes place, because the primary will fill his socalled Roche-lobe. Also mass is lost from the system. When the primary has expelled all of his hydrogen-rich shell, the remaining carbon-oxygen core will contract and cool down to form a CO WD with a mass just below the earlier mentioned Chandrasekhar mass of 1.4M_sun. The secondary however, continues to burn hydrogen and then helium. The timescale related to the evolution of this star is determined primarly to the mass of the star after the accretion phase. This time is about 10¹⁰ yr. After this time, also the secondary will evolve into a red giant, filling his Roche lobe and therefore causing a new phase involving mass-transfer, this time however from the secondary to the primary. Eventually the primary star reaches this deadly limit of 1.4M_sun. What happens at this very moment, that is excactly the big question of modern research. But it is quite certain that at this point the primary will explode very soon.

In some articles there is also raised the possibility that the exploding system consists of two WDs. In this model two WDs are so close together that they finally collide into each other, giving raise to a SN. However, in recent articles I haven't found anything about this colliding dwars model, so I think it's save to assume that this model has been eliminated as a possiblility, however I haven't been able to find any evidence for that.

It might strike you that in both models there is a very prominent role for WDs. This is no coincedence. Later on, when I'll take a look at the spectra of type IA SN it will become apparent that the lines of hydrogen are absent. So, in proposing a model for a SN IA explosion or progenitors to such an explosion, you must at least be able to account for this fact. Well, in a WD there is almost no hydrogen present, because the hydrogen was expelled during the red-giant phase. Later I'll come to more evidence for WDs as the progenitors of IA SNe, but this is one of the major reasons.
Another striking fact might be that in both models a binary system is proposed. Also this is no coincedence. Multiple researches have shown that a single star system cannot produce a SN IA-explosion and that a binary system in which mass transfer takes place during the evolution works a lot better (eg Mazurek 1973, Wheeler & Hansen 1971, Truran & Cameron 1971). This is mainly because of problems you get when you are trying to 'ignite' the explosion in an one-star system.

I.3 Proceedings

Till this point I more or less put down a general picture for the reader. I'll now proceed to a quite comprehensive survey of observed properties of SN IA explosions and thereby show what this means for this general picture.

The birth of a white-dwarf in a single-star system after blowing away the outer parts of the original star into the 'planetery nebula' picture taken from www.howstuffworks.com