An illustration of a protostar (a pre-main sequence star) surrounded by a disk of gas and dust
Credit: NASA/CXC/M.Weiss. Click here for a full size image
Credit: Arthure Billard.
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.
Media contact
Federica Sgorbissa
Sissa Medialab
Trieste
Italy
Tel: +39 40 3787644
Mob: +39 340 5473118
federica@medialab.sissa.i
Science contact
Xiaoting Fu
SISSA (International School for Advanced Studies)
Trieste
Italy
Tel: +39 040 3787 481
xtfu@sissa.it
Further information
The new work appears in “Lithium evolution in metal-poor stars: from pre-main sequence to the Spite plateau", X. Fu, A. Bressan, P. Molaro, P. Marigo, Monthly Notices of the Royal Astronomical Society, in press.
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