Extremely metal-poor stars from the cosmic dawn in the bulge of the Milky Way

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Abstract

The first stars are predicted to have formed within 200 million years after the Big Bang1, initiating the cosmic dawn. A true first star has not yet been discovered, although stars2,3,4 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 time5,6. The Milky Way bulge underwent a rapid chemical enrichment during the first 1–2 billion years7, leading to a dearth of early, metal-poor stars8,9. 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.

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Figure 1: Extracts of the spectrum of the lowest-metallicity star in our sample.
Figure 2: The Galactic positions and orbits of the 23 stars observed at high resolution.
Figure 3: Chemical abundances of the 23 stars observed at high resolution.

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Acknowledgements

This paper includes data gathered with the 6.5-m Magellan Telescopes located at Las Campanas Observatory, Chile. Australian access to the Magellan Telescopes was supported through the Collaborative Research Infrastructure Strategy of the Australian Federal Government. L.M.H. and M.A. were supported by the Australian Research Council (FL110100012). A.R.C. acknowledges support from the European Union FP7 programme through ERC grant number 320360. Research on metal-poor stars with SkyMapper is supported through Australian Research Council Discovery Projects grants DP120101237 and DP150103294 (principal investigator G.S.D.C.). The OGLE project received funding from the NSC, Poland (MAESTRO grant 2014/14/A/ST9/00121 to A.U.).

Author information

The project was initiated and led by M.A. The photometric target selection was made by L.M.H., C.I.O. and D.M.N. using data from the SkyMapper telescope developed by B.P.S., S.C.K., G.S.D.C., M.S.B. and P.T. The low-resolution spectra were obtained by L.M.H and C.I.O. The data were reduced and analysed by L.M.H. using software developed by A.R.C. Target selection for the high-resolution observations was done by L.M.H., M.A. and A.R.C. with the observations carried out by L.M.H. and D.Y.; the reduction and subsequent chemical analysis was completed by L.M.H. K.L. performed the non-LTE spectral line formation calculations, C.K. interpreted the observed chemical abundances in terms of supernova yields, and M.N. provided comparison bulge data. R.P., A.U., M.K.S, I.S., G.P., K.U., Ł.W., P.P., J.S., S.K. and P.M. obtained the OGLE observations, A.U. and M.K.S. constructed the reference images, and R.P. measured the proper motions. The manuscript was written by M.A., L.M.H. and A.R.C. with all authors contributing comments.

Correspondence to L. M. Howes.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 The C–H band of SMSS J181609.62−333218.7.

The C–H band is used to derive an upper limit for C in our most metal-poor star, SMSS J181609.62−333218.7. Synthetic spectra with abundances of [C/Fe] = 0.06 (blue) and [C/Fe] = 0.56 (red) are shown for comparison.

Extended Data Table 1 Coordinates and 2MASS photometry of the 23 stars observed
Extended Data Table 2 Stellar parameters of the 23 stars observed
Extended Data Table 3 Chemical abundances measured for each star, C to Ca
Extended Data Table 4 Chemical abundances measured for each star, Sc to Cu
Extended Data Table 5 Chemical abundances measured for each star, Zn to Eu
Extended Data Table 6 Orbital parameters

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Howes, L., Casey, A., Asplund, M. et al. Extremely metal-poor stars from the cosmic dawn in the bulge of the Milky Way. Nature 527, 484–487 (2015) doi:10.1038/nature15747

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