August 5, 2015

Lost lithium destroyed by ancient stars


‘Sunbirth’: a painting inspired by the formation of a star. Credit: Arthure Billard.

(August 5, 2015)  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.

An illustration of a protostar (a pre-main sequence star) surrounded by a disk of gas and dust.
Credit: NASA/CXC/M.Weiss.

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.

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