title: Extremely metal-poor stars from the cosmic dawn in the bulge of the Milky Way.
abstract: The first stars are predicted to have formed within 200 million years after the Big Bang, initiating the cosmic dawn. A true first star has not yet been discovered, although stars with tiny amounts of elements heavier than helium ('metals') have been found in the outer regions ('halo') of the Milky Way. The first stars and their immediate successors should, however, preferentially be found today in the central regions ('bulges') of galaxies, because they formed in the largest over-densities that grew gravitationally with time. The Milky Way bulge underwent a rapid chemical enrichment during the first 1-2 billion years, leading to a dearth of early, metal-poor stars. Here we report observations of extremely metal-poor stars in the Milky Way bulge, including one star with an iron abundance about 10,000 times lower than the solar value without noticeable carbon enhancement. We confirm that most of the metal-poor bulge stars are on tight orbits around the Galactic Centre, rather than being halo stars passing through the bulge, as expected for stars formed at redshifts greater than 15. Their chemical compositions are in general similar to typical halo stars of the same metallicity although intriguing differences exist, including lower abundances of carbon.
authors.affiliation: Research School of Astronomy and Astrophysics, Australian National University, Australian Capital Territory 2601, Australia.
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authors.initials: LM
authors.lastname: Howes
authors.affiliation: Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK.
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authors.initials: AR
authors.lastname: Casey
authors.affiliation: Research School of Astronomy and Astrophysics, Australian National University, Australian Capital Territory 2601, Australia.
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authors.lastname: Asplund
authors.affiliation: Research School of Astronomy and Astrophysics, Australian National University, Australian Capital Territory 2601, Australia.
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authors.affiliation: Research School of Astronomy and Astrophysics, Australian National University, Australian Capital Territory 2601, Australia.
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authors.affiliation: Research School of Astronomy and Astrophysics, Australian National University, Australian Capital Territory 2601, Australia.
authors.forename: D M
authors.initials: DM
authors.lastname: Nataf
authors.affiliation: Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland.|Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, Ohio 43210, USA.
authors.forename: R
authors.initials: R
authors.lastname: Poleski
authors.affiliation: Department of Physics and Astronomy, Division of Astronomy and Space Physics, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden.
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authors.lastname: Lind
authors.affiliation: Research School of Astronomy and Astrophysics, Australian National University, Australian Capital Territory 2601, Australia.|School of Physics, Astronomy and Mathematics, Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK.
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authors.initials: C
authors.lastname: Kobayashi
authors.affiliation: Research School of Astronomy and Astrophysics, Australian National University, Australian Capital Territory 2601, Australia.
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authors.initials: CI
authors.lastname: Owen
authors.affiliation: Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Ness
authors.affiliation: Research School of Astronomy and Astrophysics, Australian National University, Australian Capital Territory 2601, Australia.
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authors.lastname: Bessell
authors.affiliation: Research School of Astronomy and Astrophysics, Australian National University, Australian Capital Territory 2601, Australia.
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authors.initials: GS
authors.lastname: Da Costa
authors.affiliation: Research School of Astronomy and Astrophysics, Australian National University, Australian Capital Territory 2601, Australia.
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authors.lastname: Schmidt
authors.affiliation: Research School of Astronomy and Astrophysics, Australian National University, Australian Capital Territory 2601, Australia.|Sorbonne Universités, UPMC Université Paris 6 et CNRS, UMR 7095, Institut d'Astrophysique de Paris, 98 bis Boulevard Arago, 75014 Paris, France.
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authors.lastname: Tisserand
authors.affiliation: Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland.
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authors.initials: A
authors.lastname: Udalski
authors.affiliation: Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland.
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authors.initials: MK
authors.lastname: Szymański
authors.affiliation: Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland.
authors.forename: I
authors.initials: I
authors.lastname: Soszyński
authors.affiliation: Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland.|Universidad de Concepción, Departamento de Astronomia, Casilla 160-C, Concepción, Chile.
authors.forename: G
authors.initials: G
authors.lastname: Pietrzyński
authors.affiliation: Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland.|Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK.
authors.forename: K
authors.initials: K
authors.lastname: Ulaczyk
authors.affiliation: Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland.
authors.forename: Ł
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authors.affiliation: Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland.
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authors.lastname: Pietrukowicz
authors.affiliation: Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland.
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authors.affiliation: Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland.
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authors.lastname: Kozłowski
authors.affiliation: Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland.
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authors.lastname: Mróz