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Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud
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Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud
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Journal Article
Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud
2012
Overview
The abundance of interstellar
7
Li in the low-metallicity gas of the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun’s metallicity, is nearly equal to the Big Bang nucleosynthesis predictions.
The search for local cosmological lithium-7
The predicted primordial abundance of the lithium-7 isotope in the primordial Universe is four times greater than that measured in the atmospheres of Galactic halo stars, but it is hard to trace this isotope in the Milky Way because it has most probably been burnt up. This paper reports the detection of interstellar lithium beyond the Milky Way, in the low-metallicity gas of the nearby Small Magellanic Cloud galaxy. Present-day lithium-7 abundance in this galaxy is nearly equal to the predictions of the standard theory of Big Bang nucleosynthesis — although the data can also be reconciled with non-standard models.
The primordial abundances of light elements produced in the standard theory of Big Bang nucleosynthesis (BBN) depend only on the cosmic ratio of baryons to photons, a quantity inferred from observations of the microwave background
1
. The predicted
2
,
3
,
4
primordial
7
Li abundance is four times that measured in the atmospheres of Galactic halo stars
5
,
6
,
7
. This discrepancy could be caused by modification of surface lithium abundances during the stars’ lifetimes
8
or by physics beyond the Standard Model that affects early nucleosynthesis
9
,
10
. The lithium abundance of low-metallicity gas provides an alternative constraint on the primordial abundance and cosmic evolution of lithium
11
that is not susceptible to the
in situ
modifications that may affect stellar atmospheres. Here we report observations of interstellar
7
Li in the low-metallicity gas of the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun’s metallicity. The present-day
7
Li abundance of the Small Magellanic Cloud is nearly equal to the BBN predictions, severely constraining the amount of possible subsequent enrichment of the gas by stellar and cosmic-ray nucleosynthesis. Our measurements can be reconciled with standard BBN with an extremely fine-tuned depletion of stellar Li with metallicity. They are also consistent with non-standard BBN.
Publisher
Nature Publishing Group UK,Nature Publishing Group
Subject
/ Characteristics and properties of external galaxies and extragalactic objects
/ Chemical composition and chemical evolution
/ Exact sciences and technology
/ Humanities and Social Sciences
/ Isotopes
/ letter
/ Lithium
/ Local group. Magellanic clouds
/ Magellanic clouds and other irregular galaxies
/ Science
/ Stars
/ Stellar systems. Galactic and extragalactic objects and systems. The universe
