The 2011 Nobel prize was given to the two teams who discovered the acceleration of the Universe using Type Ia supernovae; a result supported by every other method sensitive enough to probe acceleration. However, it remains a disturbing truth that we know relatively little about Type Ia supernovae and where they come from.
The mantra that we usually trot out (and I am as guilty as anyone of this) is that they are pretty good standard candles because the progenitor is a white dwarf that accretes matter from a companion star and explodes when it reaches the Chandrasekhar limit. It is very neat and convenient.
Unfortunately there are some complicating factors. First, it appears as if there are two types of Ia’s, some of which form promptly and some of which are delayed. Secondly, it isn’t clear whether Type Ia’s predominantly form from a single white dwarf (singly-degenerate channel) with a normal star companion or from two white dwarfs (doubly degenerate).
The various problems associated with the Ia progenitors were recently summarised here. One of the most intriguing problems is that if Ia’s explode via the singly-degenerate channel by accreting steadily from a normal companion, then it would be rather natural for them to be undergoing nuclear burning prior to exploding and hence to be hot enough (kT around 100 eV) to be super-soft X-ray sources.
The problem is, if this were true and most Ia’s were doing this, they would be visible in searches for them. However, we simply don’t see enough of these sources to match the observed SNIa rate: the shortfall may be as much as two orders of magnitude! Perhaps the energy from the accretion is being released at other wavelengths or perhaps our underlying models of Type Ia’s are wrong. Whatever the reasons, these are challenges we will need to figure out if we want supernovae to remain competitive as next-generation probes of cosmology.
Update: 21 September 2012: There is another fascinating problem for the single degenerate (SD) scenario to add to the lack of super-soft sources. One of the natural questions we can ask is, “what happens to the companion star that donated the material that tipped the white dwarf over the Chandrasekhar lip and triggered the supernova?” Although the white dwarf itself it expected to be totally incinerated, the companion star does not ignite. Instead it is a passive victim of the blast wave, which strips off some of the outer layers of hydrogen. Now, the key thing about SNIa is that their spectra are completely missing both hydrogen and helium. But if the blast wave strips off too much hydrogen and/or helium from the companion, we would see it in the spectra. Observational limits on Hydrogen in SNIa explosions are currently very stringent, suggeting an upper limit of around 0.01 times the mass of our sun in Hydrogen.
In a recent paper, Liu et al did 3-D SPH hydrodynamic simulations of the blast wave interaction with the companion, finding that it stripped between 0.11 and 0.18 times the mass of our sun, apparently drastically in conflict with the observational limits.
As they point out however, there are possibilities that actually when one does much more realistic simulations, significantly less hydrogen will be stripped, or perhaps that the observational limits are overly tight, but it does certainly pose an interesting challenge to the SD scenario.