If I had to pick a handful of favourite things in all of physics, black holes and nuclear fusion would be among them. Hands down. This interesting little paper which I stumbled across earlier (via a link to the APS website posted to twitter by @ArabellaWick) uses both of these things to try and address the old puzzle about lithium abundances in the Universe.
It’s an old puzzle. Essentially, when you account for all the sources through which the universe is capable of producing Lithium atoms (a process called nucleosynthesis), including the Big Bang itself, you can get a fairly good estimate of how much lithium there should be in, in total, in the Universe right now. But the Universe doesn’t want to play ball. We see between three and four times less lithium out there than we should (which is a discrepancy of about 4-5 σ). In other words, there are processes out there in the Universe involving lithium, which we don’t really know or understand.
Lithium is interesting because it’s difficult to make naturally. It’s destroyed by the nuclear fusion which occurs in stars, and even more so if those stars host planets. In fact, it’s generally thought that the only way to efficiently create lithium is cosmic ray spallation (despite the authors in this paper referring to it as a process by which lithium is destroyed). This paper’s contribution to this conundrum is to present a new method in which lithium can be formed. Which seems slightly baffling at first, given that the problem is an underabundance – until the authors point out that they’re not trying to address the lithium problem, but add another piece to the puzzle. And it’s an interesting piece.
Stars are generally considered to be the element factories of the Universe. Stellar fusion creates the elements we’re all made from and continues to provide us with life giving sunlight. Stellar fusion processes require a lot of energy and give a lot of energy out, but those energies pale in comparison to those found in the environments surrounding black holes.
Black holes are the most incredibly extreme objects in the universe, and the tidal forces they command put unimaginable stresses on any material in orbit around them. Black holes may typically be found with a spiralling disk of material surrounding them, and extreme gravity in such a disk (also known as a torus) can convert matter to energy more efficiently than any star can. Which is really saying something, believe me.
To cut a long story short, this paper explains in some rather nice detail how a black hole torus can reach extremely high temperatures – easily sufficient for nuclear reactions requiring threshold energies of over 10 MeV (mega electronvolts) to occur. In particular, 74 different nuclear reactions were found to occur under these conditions. The authors describe 17 as being photodissociations – though personally I’d argue that photodissociation is a chemical process, and that photodisintegration would be a more appropriate term.
The particular nuclear reactions are apparently 4He(α,n)7Be, 4He(α,p)7Li, 4He(α,d)6Li and 4He(α,n p)6Li. If I’m perfectly honest, I find this surprising. I was previously entirely under the impression that such reactions didn’t tend to occur. Lithium and beryllium have a tendancy to be destroyed during stellar nucleosynthesis reactions.
EDIT – My big puzzle in this paper is that 7Be is unstable and 6Li and 7Li are stable but destroyed in stellar fusion – specifically by proton capture. The cycle normally ends with helium being produced, which is the opposite of what this paper is concluding. So after chatting with a tame particle physicist by the name of @article82, the crux of the matter seems to be that at higher temperatures, protons can be removed from atomic nuclei by photodisintegration faster than they can be be captured. Combined with the authors of this work expecting the lithium created to be expelled from the black hole’s surrounding torus. Not a huge leap of reasoning, given the high energies imparted to recently fused nuclei.
In closing, the authors discuss how the mechanism they propose could produce amounts of lithium comparable with other forms of nucleosynthesis, such as the Big Bang or cosmic rays. They suggest that regions near black holes could be searched for lithium, perhaps in stellar systems like Cygnus-X1 . Lithium, incidentally, is fairly easily picked out in the optical light I’m personally more familiar with, by a telltale spectroscopic feature at 670.8 nm. Interestingly, companion stars in systems believed to contain black holes or neutron stars have already been observed to contain anomalously high levels of lithium.
All in all, I love this little piece of research. Getting the idea into peoples heads of black holes creating things as well as destroying them is quite beautiful. Just think. If a black hole can create lithium, then – even inspite of how infrequently the triple alpha process happens – it can also create carbon, oxygen and other light elements. Perhaps some of the atoms in your body came from a black hole!
Fabio Iocco, & Miguel Pato (2012). Lithium Synthesis in Microquasar Accretion Phys. Rev. Lett., 109 (2) DOI: 10.1103/PhysRevLett.109.021102