The COMBS survey I: Chemical Origins of Metal-Poor Stars in the Galactic Bulge

Madeline Lucey, Keith Hawkins, Melissa Ness, Martin Asplund, Thomas Bensby, Luca Casagrande, Sofia Feltzing, Kenneth C. Freeman, Chiaki Kobayashi, Anna F. Marino

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Chemistry and kinematic studies can determine the origins of stellar population across the Milky Way. The metallicity distribution function of the bulge indicates that it comprises multiple populations, the more metal-poor end of which is particularly poorly understood. It is currently unknown if metal-poor bulge stars ([Fe/H] <−1 dex) are part of the stellar halo in the inner most region, or a distinct bulge population or a combination of these. Cosmological simulations also indicate that the metal-poor bulge stars may be the oldest stars in the Galaxy. In this study, we successfully target metal-poor bulge stars selected using SkyMapper photometry. We determine the stellar parameters of 26 stars and their elemental abundances for 22 elements using R∼ 47 000 VLT/UVES spectra and contrast their elemental properties with that of other Galactic stellar populations. We find that the elemental abundances we derive for our metal-poor bulge stars have lower overall scatter than typically found in the halo. This indicates that these stars may be a distinct population confined to the bulge. If these stars are, alternatively, part of the innermost distribution of the halo, this indicates that the halo is more chemically homogeneous at small Galactic radii than at large radii. We also find two stars whose chemistry is consistent with second-generation globular cluster stars. This paper is the first part of the Chemical Origins of Metal-poor Bulge Stars (COMBS) survey that will chemodynamically characterize the metal-poor bulge population.

Original languageEnglish
Pages (from-to)2283–2300
Number of pages18
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date8 Jul 2019
Publication statusPublished - 1 Sept 2019


  • astro-ph.SR
  • astro-ph.GA
  • Stars: abundances
  • Galaxy: evolution
  • Galaxy: bulge
  • Stars: Population II


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