Lithium, the lightest metal, used in batteries and mood-stabilising drugs, is rarer than it should be. Models of the period after the Big Bang explain how it, hydrogen and helium were synthesised in nuclear reactions, before the universe cooled enough for the stars and planets that we see today to come into being. Astronomers though think that about three times as much lithium was produced in that earliest epoch than remains today in the oldest stars in the galaxy, and the difference has proved hard to explain.
Now a group of scientists, led by Xiaoting Fu of the International School for Advanced Studies in Trieste, Italy, think they have the answer to this so-called ‘lithium problem’: it was destroyed and re-accumulated by these stars shortly after they were born. The team publish their work in Monthly Notices of the Royal Astronomical Society.
In the past astronomers have speculated on what might be responsible for the lithium deficit. Ideas included as yet unknown aspects of particle physics, nuclear physics or even new models of cosmology.
Fu’s team instead looked at how much lithium there would have been when a particular subset of the first long-lived stars formed, just a few hundred million years after the Big Bang. These are still around today, so provide astronomers with some insight into the history of the universe and how its composition has changed.
The stars have between 50 and 85% of the mass of the Sun, have lives that are significantly longer, and are thought to remain stable on the so-called ‘main sequence’ for between 15 and 30 billion years. They are poor in most ‘metals’, which in astronomy means every element heavier than helium. The scientists modelled the way that these stars process lithium, starting with the early part of their lives when they are still contracting and heating up under the influence of gravity.
In that ‘pre-main sequence’ phase, the new model suggests that there is more mixing in the different layers of these objects. To put this in context, stars have a hot core, where nuclear fusion is converting hydrogen to helium, a cooler outer layer where convection cycles material from above the core to the surface and down again, and a surface where electromagnetic radiation (including light and heat) escapes into space.
The new work indicates that in this first phase of their lives, the low-mass stars have an extra mixing ‘overshooting’ at the base of the convection zone, where surface lithium is brought to the hot interior and almost completely destroyed.
Pre-main sequence stars are also surrounded by the residual gas and dust from which they formed. This cloud will over time be pulled on to the star, adding lithium to its surface. As the star ages, the convective zone becomes shallower, so material is no longer sent to the core, to some extent offsetting the earlier destruction of lithium.
Stars also shine brightly in ultraviolet light, and the ‘radiation pressure’ of this light eventually blows the disk materials away, stopping the star from accumulating more lithium. The stars then enter the main sequence and settle into a long period of stability. When we observe them now, between 10 and 12 billion years later, they show a constant abundance of lithium, which is about one third of the primordial level.
Fu comments: “Our work is a completely new approach to the lithium problem. The model not only may explain the loss of lithium in stars, but could also help explain why the Sun has fifty times less lithium than similar stars and why stars with planets have less lithium than stars on their own.”
In the next decade new observatories like the European Extremely Large Telescope (E-ELT) under construction in Chile should allow astronomers to look back at the first metal-poor stars as they formed, and confirm the rapid loss of lithium in the early Universe.