Causing quite a big stir in the astrochemical world recently is the astronomical detection of C60, more popularly known as fullerene. This is kind of a big deal. Fullerenes have been known about since the 1980s when Harry Kroto et al first created them by vapourising graphite with electrical discharges. Since then, it’s been found that C60 molecules are very stable and readily formed molecules. Lots of people have hypothesised them existing in space, but up until now there’s been no conclusive evidence*. And believe me, in science, it’s all about finding conclusive evidence**.
C60 is formed readily in the right conditions. So readily in fact, that any other purely carbon molecules try to form themselves into C60 too. Give a mixture of carbon vapour the right conditions for long enough, and fullerenes will eventually be the only molecules in it. That said, the formation of these little carbonaceous footballs is inhibited by hydrogen, and hydrogen makes up a huge percentage of the baryonic matter in the Universe. So for fullerenes to form in space, you need an environment which is lacking in hydrogen. And environment a bit like the planetary nebula Tc 1.
Tc 1 is interesting. As the authors of this paper note, it’s not necessarily an unusual object, though it may be going through an unusual phase in its life. It’s a carbon rich pre-planetary nebula, one of the classic hunting grounds for interesting molecules. As far as they can tell, the central star in this nebula underwent a thermal pulse a few thousand years ago, expelling the star’s outer envelope. What remains is a hydrogen-poor environment in the middle of the star. So hydrogen poor is this region, that none of the usual hydrogen-containing molecules have been found there! No acetylene. No hydrogen cyanide. No polycyclic aromatic hydrocarbons. Seemingly, this makes for a prime formation site for fullerenes, which dominate the infrared spectra of the object. They go on to talk about how they believe these fullerenes to be present on the surface of dust grains and make a few justifications which… I don’t necessarily agree with. But perhaps controversially, I’m not actually going to talk about that.
Perhaps the most interesting thing here is the lesson to be learned for all people such as myself who go hunting for spectral fauna. This isn’t the first time an infrared spectrum of Tc 1 has been taken. It isn’t even the first time it’s been published. It is, however, the first time it’s been recognised as arising from fullerenes. To a chemist, the characteristic four-peak spectrum of C60 is readily identifiable and quite obvious. To an astronomer, however, there’s no immediate reason why it should be much more significant than any other multiple-peaked mid-infrared spectrum. It serves to underpin the fact that we should all check the things we publish very closely, and be wary for what we might miss. I rather wonder how many other things lie undiscovered in spectra that have been published and looked over many times before.
Interestingly, this isn’t the first time I’ve actually seen a convincing detection of fullerenes in the interstellar medium. At conference I was attending in France a couple of months ago, Kris Sellgren presented a rather convincing detection of C60.*** It certainly convinced me, anyway. I can only assume, however, that she and her collaborators didn’t publish anything on it in time, because there’s no reference made to it in this paper at all. The only reference to Sellgren is in a line about inconclusive detections. Perhaps this is also a warning that if you have an exciting discovery, while it’s good to double check things to make certain you’re correct, it’s also a good idea not to spend too long in doing so…
It’s funny how a paper can sometimes teach you a lot more than just the information it contains.
*On a similar note, during a recent conference, Kroto quipped that if no one found a good use for fullerene soon, he might have to hand his nobel prize back!
**Two footnotes in the same paragraph? Blimey… Actually, as conclusive evidence goes, one of the other molecular detections mentioned in this paper is the previous detection of benzene. Frankly, I’ve read that paper, and the result is far from conclusive! They also mention the relation of C60+ to a couple of diffuse interstellar bands in the near-infrared. In this case, instead of conclusive the usual phrasing tends to be “not inconsistent with”…
***I’m going to reserve comment on the fact that at least two authors on this paper were also at the same conference (I know because I attended the talks they gave). Having never been in such a position myself, I can’t really imagine what must go through one’s mind on watching someone else present work clearly in direct competition with your own. Though speaking purely for myself, I’d have included at least some acknowledgement of Sellgren’s presentation…
Cami, J., Bernard-Salas, J., Peeters, E., & Malek, S. (2010). Detection of C60 and C70 in a Young Planetary Nebula Science DOI: 10.1126/science.1192035