The Milky Way is not special: accreted stars also inhabit the Spite Plateau

Speaker(s) Jeffrey Simpson, UNSW Sydney

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The ESA Gaia astrometric mission has enabled the remarkable discovery that a large fraction of the stars within a few kiloparsecs of the Sun appear to be debris from a single in-falling system, the so-called Gaia-Sausage-Enceladus. One exciting feature of this result is that it gives astronomers for the first time a large sample of easily observable, unevolved stars that formed in an extra-Galactic environment, which can be compared to stars that formed within our Milky Way. In this talk I will discuss using these stars to investigate the “Spite Plateau” – the near-constant lithium abundance observed in metal-poor dwarf stars across a wide range of metallicities (-3 < [Fe/H] < -1). In particular our aim was to test whether the stars that formed in Gaia-Sausage-Enceladus show a different Spite Plateau to Milky Way stars that inhabit the disk and halo. Individual galaxies could have different Spite Plateaus – e.g., the interstellar medium could be more depleted in lithium in a lower galactic mass system due to it having a smaller reservoir of gas. We find that the Gaia-Sausage-Enceladus stars show the same lithium abundance as other likely accreted stars and in situ Milky Way stars, strongly suggesting that the “lithium problem” is not a consequence of the formation environment. This result fits within the growing consensus that the Spite Plateau, and more generally the “cosmological lithium problem” – the observed discrepancy between the amount of lithium in warm, metal-poor dwarf stars in our Galaxy, and the amount of lithium predicted to have been produced by Big Bang Nucleosynthesis – is the result of lithium depletion processes within stars.