Why do type 1a supernovae appear dimmer than expected?

[Chalnoth]

In that case it's either a hydro effect, asphericity effect, or a radiation effect. Having a three-d simulation that explodes when a 2-d doesn't, isn't useful. You should be able to run the 3-d simulation and explain why 3-d causes a difference and then work that back into a 1-d code.

Nope, because the explosion in those cases arises from asymmetric instabilities, which cannot be modeled in one dimension.

I need to review the literature on type II's over the last year to see if someone has come up with something new, but off the top of my head I don't see how this is going to work. If you have instabilities that affect the shock itself, then you are hosed because you don't have nearly the resolution to see the shock itself. If you have the instabilities develop behind the shock then you have the problem that I mentioned earlier.

Basically, from the simulations I've seen, what happens is that small oscillations lead to oscillation of the shock front along one axis (e.g. up/down). Those oscillations then grow until the shock front is destabilized and the supernova explodes.

One big problem with full three-d simulations is that if you have very detailed hydrodynamics, then most of the time they are using much less detailed neutrino physics, and if you use less detailed neutrino physics, it's not obvious that the explosions that you are getting are the result of having a crude neutrino algorithm that reduce losses. If you are using 3-d hydro but 1-d neutrino physics, it's pretty easy to come up with a calculation that is inconsistent.

From what I can tell, this is a big reason why we still don't know what's causing the explosions.

 

[twofish-quant]

Nope, because the explosion in those cases arises from asymmetric instabilities, which cannot be modeled in one dimension.

If it's something like SSAI, then once you understand the basic physics, you can insert it into a 1-d code either by incorporating a neutrino enhancement term or by including a drag term behind the shock. The strategy is to use 3-d models to tune the parameters for a 1-d model that has the basic physics. You can then use the 1-d model for things like nucleosynthetic calculations.

If it's a low dimensional axisymmetric instability then you can incorporate that sort of physics in 1-d.

Basically, from the simulations I've seen, what happens is that small oscillations lead to oscillation of the shock front along one axis (e.g. up/down). Those oscillations then grow until the shock front is destabilized and the supernova explodes.

And I'll be skeptical that this really does solve the explosion problem until someone explains how it overwhelms the stabilizing effects of neutrino losses. This is a *very* interesting line of research, but since the 1970's, the story of type II supernova modeling has been one where you have an interesting effect that turn out not to work after you put in more realistic neutrino physics. It doesn't help that people aren't completely sure that energy is conserved and that there are at least three mechanisms for what might going on (SASI, acoustic coupling, and MRI).

So I'm not breaking open the champagne yet. :-) :-)

From what I can tell, this is a big reason why we still don't know what's causing the explosions.

The basic problem with type II supernova is that there are about ten different things that are going on, all of which may interact with each other in very complicated ways.

 

[Chronos]

I'm fairly confident there are no conservation of energy issues, tq, just confused mathematical models.

 

[Garth]

I can't remeber who it was, but I saw a talk recently by someone who'd done some interesting work on assymmetric SN1a explosions, caused I think by some larger than normal lump accreting onto the white dwarf just as it crosses the Chandresarkar limit. The claim was that the simulation gave you an abnormal SN1a, but that the results matched well to some known 'unusual' 1a's. It was an interesting talk, but it was clear that a lot of work remained to be done.

Was the talk be similar to this discussion? Nebular Spectra and Explosion Asymmetry of Type Ia Supernovae

Garth

 

[twofish-quant]

I'm fairly confident there are no conservation of energy issues, tq, just confused mathematical models.

I'm not so sure. Getting global energy to conserve in a hydro-simulation is much, much harder than it seems. There is a strong likelihood that the first time you run the code, it *won't* conserve energy and you'll be spending a month trying to figure out why.

Even trying to check if you've conserved energy in a 3-d Euler simulation is extremely non-trivial. You have matter falling into the simulation. You have radiation leaving the simulation, you have non-trivial matter-radiation interactions.

One problem with hydro simulations is that even very, very slight differences in energy balance will cause reasonable but incorrect results. It's not the huge bugs that you worry about, since huge bugs are obvious. It's the subtle complicated ones that keep you up at nights. There is also the fact that all numerical codes will have bugs. This is why "run the simulation and declare victory" won't work.

In any case, I won't feel confident in breaking out the champagne until you have three or four groups with different simulations and code bases, come up with the same basic mechanism for an explosion.