Too Much, Too Little: A Tale Of Cosmic Lithium
Two recent studies find more and less lithium than expected in very different places in the Universe.
(Solid metallic lithium floating in oil. Credit: Wikipedia)
Lithium is the lightest naturally-occurring element found in a metallic state on Earth; only hydrogen and helium are lighter, and both are gases at our temperatures and pressures. It’s a tiny constituent of our world, only 0.0017% by number. But it’s a chemical oddball in nature, even on as local a scale as the solar system. Astronomers studying the abundance of elements in the atmosphere of the Sun found it many times lower than the ‘primordial’ abundance in meteorites — thought to be the starting abundance of elements in the interstellar cloud out of which condensed the Sun and the planets. Later, we found that it’s oddly absent, or at least severely depleted in many stars. Recently, it was determined that stars known to host exoplanets are especially depleted in lithium, suggesting that the presence of lithium is some kind of indicator of the likelihood that a stellar system will form planets. The reason stars appear to “lose” their lithium is that under the right conditions it is destroyed by nuclear fusion, turned into heavier elements in the hot, deep interiors of young stars. However, this process is thought to occur only early in the lives of stars, setting their fraction of lithium at a fixed, permanent level.
Stars don’t really make much, if any, lithium. Instead, much of the lithium in the Universe was probably made in the Big Bang, which had just enough energy to create some of the lightest “metals” (to astronomers, elements heavier than helium). But we haven’t accounted for all of it. They key is which kind of lithium we’re talking about; that means which isotope. Kinds of atoms are defined by their atomic number, equal to the number of protons in the atom. Hydrogen has one, helium two, lithium three, and so on throughout the periodic table. The number of neutrons in an atom, however, doesn’t determine the kind of atom, just how heavy it is. The problem is with lithium-7, the most common form of lithium in the Universe: it has three protons and four neutrons. Some 30 years ago, researchers found apparently too little lithium-7 in the atmospheres of certain old stars, in contradiction to models of the amount of lithium made in the Big Bang; this became known as the “lithium problem”. The problem is sort of embarrassing for people who work on the abundances of elements in the Universe, because we seem to understand the other elements much better. Two new studies are further complicating the story.
(The Small Magellanic Cloud — the smaller of the two fuzzy dots to the right of the Milky Way’s dusty disk — is a dwarf galaxy currently orbiting the Milky Way. Credit: S&T Image Gallery: Luis Argerich)
Recent observations of stars in the Magellanic Clouds, small companion galaxies orbiting our Milky Way, shows the low abundance of lithium isn’t going away. J. C. Howk and co-workers used the high-resolution UVES spectrograph on the 8.2-meter Very Large Telescope in Chile to record the spectrum of a bright, young star in the Small Magellanic Cloud. They used it like a distant lightbulb to illuminate the gas and dust in the galaxy, allowing them to measure how many lithium atoms along the line of sight to the star were absorbing its light. They found that the amount of lithium is still less than what Big Bang models predict should have been around when the SMC formed. The problem is that some small amount of lithium is made inside stars, so the starting amount of lithium in the SMC had to be lower than it is today — in even greater disagreement with Big Bang predictions.
(The globular star cluster Messier 4. Credit: European Southern Observatory)
Sounds like Big Bang is doomed, right? One of its core predictions seems to not be verified by our observations of stars. It gets weirder. Another new paper claims to have found precisely the opposite — an old star in an ancient star cluster that seems to have way too much lithium. Observations of the globular star cluster Messier 4 show that at least one star — called by the designation “37934” — has far more lithium than both other stars in the cluster and the Big Bang prediction. The authors caution that this apparent overabundance may be the result of star 37934 happening to incorporate the lithium “pollution” of many earlier generations of stars by chance, but they cannot rule out the possibility that in fact the Big Bang lithium fraction was simply higher than the models predict. That may mean there’s something imperfect about our understanding of the physics of the Big Bang.
There’s a twist in this story, and it seems to back up the idea that we don’t really know as much about the Big Bang as we think. Remember the Small Magellanic Cloud? Howk’s team also may have seen the subtle fingerprints of a lighter, rarer isotope of lithium — lithium-6 — in their data. They’re not confident enough in the data to claim a detection, but if their result is later verified by other observers, the implication is amazing: there’s about a thousand times less lithium-6 in the SMC than the Big Bang models predict. The Howk group has more telescope time scheduled this fall to find out.
How much is the “right” amount of lithium in the Universe? Do we really see too much, or too little? The answers may be coming soon.