Are novae producing the Milky Way’s lithium?
Astronomers don’t fully understand the lithium abundances of the Milky Way. An exploding white dwarf called Nova Centauri 2013 might be able to help.
Lithium. You might know it as a key part of the batteries that power your new digital watch and make Boeing 787s catch fire. If you’re up on your cosmology, you might recall that it was one of three elements produced in the Big Bang, along with hydrogen and helium, although we find our lithium in a much less exciting way: brine pools and igneous rocks.
For astronomers, the third-lightest element is a peculiar creature. It’s made in some stars and destroyed in others; measuring the abundance of lithium in a low-mass star can differentiate red dwarfs from brown dwarfs — a method known as the lithium test. But there’s a major problem with lithium: there’s not enough of it. Standard models of Big Bang nucleosynthesis (BBN), the pathways by which the first atoms were created, predict the amount of lithium we should see locked up in celestial objects. For decades, though, observations have shown that there’s only a third as much as theory claims.
This discrepancy is known as the lithium problem, and it’s been a major topic of research for decades. However, there’s a second issue, too: some young, metal-rich stellar populations don’t have too little lithium — they have too much of it. This throws another wrench into any coherent understanding of how lithium is produced and destroyed in the universe. I’m going to focus on one possible way out of the problem of the overabundance of lithium in certain stars: with novae, thermonuclear explosions on white dwarfs. The runaway fusion isn’t strong enough to tear the star apart, like Type Ia supernovae, but it’s significant nonetheless.
During the explosion, could obscure fusion mechanisms produce enough lithium to account for the lithium discrepancy? Recent observations say they might. We only have to go back a few years to find one of the latest data points, an eruption designated Nova Centauri 2013. But to fully understand the theory behind it, we have to go way back to the 1970s.
Back to basics: Big Bang nucleosynthesis
Actually, before we put on our bell bottoms and travel back in time, let’s try to understand why the lithium problem is, well, a problem. In the first second or so after the Big Bang, the universe was awash in protons in neutrons, bathed in a sea of radiation. With a temperature of 1 MeV, there were now enough photons to prevent deuterium from forming; when the temperature fell to about 100 keV, heavier elements could form — starting with deuterium. After the first synthesis of ³He and ⁴He, ⁷Be could form, which decayed to ⁷Li after the universe cooled even further. In this way were the lightest elements forged: a frenzied mixture of nuclear reactions, buoyed by the strong nuclear force in a rapidly expanding and cooling universe.
Theoretical abundances — including ratios of different isotopes of a given element — can be computed using various numerical models of Big Bang nucleosynthesis. Observational astronomers can test those models in a number of different ways: looking at high-redshift gas clouds to determine deuterium abundances, scouring the interstellar medium for ³He, and searching for ⁴He in metal-poor extragalactic H II regions. For hydrogen and helium isotopes, things are simple enough.
Lithium requires us to search the atmospheres of metal-poor Population II stars in the Galactic halo. Most of it is in the form of ⁷Li, but about 5% is actually ⁶Li, a difference that can only be noticed with very high-resolution spectroscopy. Observations of these stars show that the ratio of lithium to hydrogen is about one part in ten billion — too low by a factor of roughly 2–4. This isn’t insignificant, either — it’s a 4- or 5- σ discrepancy! This, then, is what we term the lithium problem, which has plagued astronomers for decades upon decades.
Independently of all this, astronomers discovered that some younger, metal-rich stars seem to have too much lithium. This is the key feature of the conundrum we’ll be trying to address. It shows that we can’t solve the lithium problem by simply adjusting our BBN models to produce more lithium; we’d still be left with these even more abnormal lithium-rich stars. Clearly, we still don’t have an accurate understanding of Galactic lithium production after the Big Bang — and I’m going to talk about one facet of that.
Laying the theoretical framework
Okay, back to the ’70s — a time when the lithium problem was already well-known. In 1975, Arnould and Nørgaard investigated whether particular heavy isotopes of lithium and beryllium (including ⁷Li and ¹¹Be) could be produced in thermonuclear explosions in stars. They actually had a variety of different environments in mind, from core collapse supernovae to explosions in the outer layers of red giants to novae on white dwarfs. The pair produced complex models involving elements as heavy as magnesium, and showed that, yes, the desired isotopes of lithium and beryllium could be produced in significant quantities.
Arnould and Nørgaard’s models were excellent, but they were intended to be rather general, valid for all three scenarios discussed above. Any definitive conclusions about, say, ⁷Li production in red giants should take into account the very specific conditions in red giant atmospheres. Within a few years, groups had worked to deal with the special cases individually (Starrfield et al. 1978). Theoreticians seemed to indicate that some proposed sites of ⁷Li production, coupled with BBN models, would produce the incorrect amounts. Therefore, Starrfield et al. turned to novae as another possibility.
In a nova, material in the outer layers of a white dwarf is ejected by runaway thermonuclear fusion involving oxygen, carbon, and other heavy elements. Temperatures can reach upwards of 150 million Kelvin, and ⁷Be is quickly formed. Some of it will be destroyed, and some will be carried by convection to cooler layers. A key reaction the beryllium may undergo is the production of ⁷Li through electron capture by ⁷Be and the emission of an electron antineutrino — the main process by which our lithium is formed.
Building on earlier numerical codes, Starrfield et al. added reactions that could produce and destroy ⁷Li, ⁷Be, ⁸B, ⁸Be, and just about any other relevant isotope they might care about. Running six separate models, they found that depending on the precise conditions, including the abundances and carbon and nitrogen at the start of the explosion, novae throughout the Milky Way might indeed produce the right amount of ⁷Li to solve the problem of too much lithium in young stars.
A discovery four decades in the making
Fast forward to 2013. In early December, an amateur astronomer named John Search notices a new magnitude +5.5 object in the constellation Centaurus. It brightens over the next couple of weeks, eventually becoming visible to the naked eye and emitting gamma rays. Spectroscopy confirms that it is, in fact, a nova. While no distance measurement can be made, astronomers are sure that it’s within the Milky Way, and designate it V1369 Cen — also known as Nova Centauri 2013.
Now, ⁷Li has a spectral line at 6708 Å, and conveniently, astronomers were able to detect an absorption feature from V1369 Cen at 6695.6 Å — the same line, but ever so slightly blueshifted, thanks to the motion of the source relative to Earth (Izzo et al. 2015). The team looked at multiple optical spectra, taken at both the La Silla and Pontiﬁcia Universidad Catolica observatories within weeks of the discovery, and concluded that the line could only be due to the nova, not some other astronomical interloper, like diffuse interstellar bands or even other metals in the ejecta masquerading as lithium.
The astronomers were able to find abundance ratios for lithium, potassium, and chlorine. Assuming the white dwarf composition was largely carbon and oxygen, they derived a hydrogen ejecta mass of about 10⁻⁴ solar masses, and a lithium ejecta mass of roughly one millionth of that — essentially, a glob of lithium the mass of the asteroid Pallas. That might not seem like a lot, but it’s enough to match Earth’s lithium production rate for over two trillion years!
Even more exciting is that given the known nova rate in the Milky Way, lithium production purely by novae accounts very well for the Galactic lithium abundance. Nova Centauri 2013 might actually be solid evidence of a solution to one of the biggest problems of lithium abundances — and all thanks to an asteroid’s worth of lithium in each eruption.