Wednesday, February 13, 2008

Making Nickel

ResearchBlogging.orgMost people are familiar with Carl Sagan's famous reminder that we are all made of "star stuff". In a previous post I discussed a 1957 paper from Burbidge, Burbidge, Fowler, and Hoyle which detailed the mechanism by which heavy elements are built up in supernovae. In that post, I concentrated on how the elements are made, but what I didn't mention is just how much of them are produced or why it's important.

Aside from the obvious reason it's important that I opened with, the elements produced also allow us to understand something about the supernovae that produce them. If you've never seen a light curve for a supernova, it rises to a peak brightness within a few days, followed by a period of rapid decay, which then shallows becoming less steep after another few days.

The change in brightness is a bit odd and can't be explained by just expansion and cooling. Instead, the light that is still being given off is a result of decay of the radioactive isotopes that are built up during the supernova. In these supernovae, a large amount of the isotope 56Ni is created. It then decays to 56Cobalt, and finally to 56Fe as (partially) illustrated in this image.

However, this is just a rough sketch. While the amount time each phase lasts is dictated by the half life of the elements, what's not fixed is the change in brightness. This will depend on how much of the 56Ni is created. Knowing how much can be made is especially important since two recent supernovae have been a bit... odd. For a typical core collapse supernova, the amount of Ni required is less than ~15% the mass of the Sun. But, supernovae 1999as and 2006gy were both much brighter than the average supernova which would obviously require that these stars be extra massive and produce a substantial amount of 56Ni to get the ball rolling. 1999as would require at least 4 times the mass of the Sun to be able to explain the light curve, and 2006gy would need at least 13 solar masses!

But can massive progenitor stars really produce that much 56Ni, especially given that there's a theoretical upper limit to how big the progenitor star can be?

This is the question posed by a recent paper published at the beginning of the month in the ApJ. Using their model, they looked at how much could be produced for different progenitor masses.

It turns out that getting the 4 solar masses required for SN 1999as isn't all that difficult. It takes a really massive star to do it, but nothing unrealistically large. Depending on just how efficient the explosion was and how much energy it gave off, it could be done with as little as ~34 solar masses. This may seem like a lot, but keep in mind there are far larger beats out there like Eta Carinae which is somewhere over 100 solar masses.

However, to get the amount of material required for 2006gy was much harder. From their model, the progenitor star would have to beat at least 200 solar masses! So what's up with this? There's several different options at this point (sorry creationists, "Magic man" isn't one of them). It could be that this supernova wasn't the typical kind we understand really well. The authors suggest that if the core were instead composed primarily Carbon and Oxygen just before collapse, the larger amount of Ni could be produced with as little as 60 times the mass of the Sun. It could also be possible that two massive stars merged. Or perhaps the theoretical upper limit isn't really there and such gigantic monsters really did exist. And of course, the possibility is always out there that the model they're using could be flat out wrong.

At this point, it doesn't look like much work has been done on figuring out 2006gy, but given how exceptional it was, being the brightest supernova yet recorded, I expect we'll see more on this beastly blast in upcoming years.

Umeda, H., Nomoto, K. (2008). . The Astrophysical Journal, 673(2), 1014-1022. DOI: 10.1086/524767

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