Making Dust

In my last post I discussed a paper in which the authors investigated the ability of supernovae to make enriched elements. While it's certainly true that supernovae are very good at this, it's certainly not all they can make.

Aside from making all the heavy elements, they make a smattering of lighter ones as well. Just after the supernova, the gas is still extremely hot and ionized. But as it cools, the electrons settle back down in their oribitals, and eventually, even complex molecules can form. These molecules are typically called "dust" although they can also take the form of organic compounds (note: "organic compounds" just refer to molecules with covalently bonded carbon and don't imply life).

Although hundreds of supernovae are discovered each year, they're almost all extremely far away. Supernova 2005X was big news in the astronomical community because it was in the relatively nearby galaxy M100. But even that is so far away that studying the expanding shell is impossible.

The last really good one that we could study was supernova 1987A which happened in the Large Magellanic Cloud (a satellite galaxy to our own). This supernova has been suggested to have begun to form dust. Additionally, a supernova that was first observed in the 1671 in the constellation of Cassiopeia (called Cas A) is also thought to be forming dust. Although there was a claim in 2003 to have located this dust, it was refuted in a Nature paper in 2004, which suggested that the dust the 2003 observers detected was actually just a large cloud between Cas A and the Earth.

But a paper that came out in January suggests that the long sought after dust from Cas A may have finally been detected.

By looking at the spectra of several regions of the nebula. Several unique spectra were found, which left the researchers to try to figure out what was causing them. Although I painted a nice rosy picture of determining chemical composition in my post on spectra, the picture becomes mush more complicated when molecules get involved because photons don't only come from transitions between orbitals, but energy can also come from vibrational and rotational modes in molecules. Additionally, there's way more types of molecules than elements.

So trying to figure out what the dust is made of is no easy task. To do it, astronomers start with what elements we know are common, and try to figure out what sorts of molecules they're likely to form. By adding the emissions from several different common species together via superposition, they attempt to recreate the observed spectra through modeling. Of course, not only do they have to figure in the chemical makeup of the dust, but they also need to try to determine the temperature since molecules act as (imperfect) blackbodies.

By fitting these spectral models to the observed spectra, it was determined that the dust formed in Cas A suggested that it has formed SiO₂, Mg protosilicates, and FeO in the inner region. Further out, they found Al₂O₃ and carbon grains. Towards the edge, they determined that the remnant had formed MgSiO₃, and either Al₂O₃ or Fe grains.

The total mass of all this junk? About 2-5% the mass of the Sun. That doesn't sound like a whole lot, until you remember that the entire mass of the Earth is only 0.00025% the mass of the Sun. So one supernova can produce enough dust to make an entire solar system worth of planets!

But the real question the paper asks is whether or not supernovae like this could produce enough dust to explain observations in distant, dust filled galaxies. They concluded that the amount of dust that supernovae like Cas A aren't nearly sufficient. In fact, they're about an order of magnitude too small. Thus, to really explain all that dust will likely require a different explanation.

Given that stars in the distant universe were much more massive than ones today, it's not unreasonable to assume that the supernovae resulting from these stars would produce much more dust as well. Unfortunately for astronomers but fortunately for all life on Earth, no such giants have exploded near us recently to be able to figure out just how much dust more massive supernovae would really produce.

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Rho, J., Kozasa, T., Reach, W.T., Smith, J.D., Rudnick, L., DeLaney, T., Ennis, J.A., Gomez, H., Tappe, A. (2008). . The Astrophysical Journal, 673(1), 271-282. DOI: 10.1086/523835