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

• Published in: Nature (London) Vol. 527; no. 7579; pp. 484 - 487
• Main Authors: Howes, L. M., Casey, A. R., Asplund, M., Keller, S. C., Yong, D., Nataf, D. M., Poleski, R., Lind, K., Kobayashi, C., Owen, C. I., Ness, M., Bessell, M. S., Da Costa, G. S., Schmidt, B. P., Tisserand, P., Udalski, A., Szymański, M. K., Soszyński, I., Pietrzyński, G., Ulaczyk, K., Wyrzykowski, Ł., Pietrukowicz, P., Skowron, J., Kozłowski, S., Mróz, P.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 26-11-2015; Nature Publishing Group
• Subjects: 639/33/34/863; 639/33/34/867; Carbon; Cosmology; Galaxies; Helium; Humanities and Social Sciences; Kinematics; letter; Metals; Milky Way; multidisciplinary; Observations; Orbits; Science; Sciences of the Universe; Stars; Stars & galaxies; Supernovae; Astrophysics - Astrophysics of Galaxies; Astrophysics - Solar and Stellar Astrophysics
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/nature15747

The first stars and their immediate successors should be found today in the central regions (bulges) of galaxies; old, metal-poor stars have now been found in the Milky Way bulge, including one star with an iron abundance about 10,000 times lower than that of the Sun without noticeable carbon enhancement, making it possibly the oldest known star in the Galaxy.
The search for the earliest stars
Galaxy formation models predict that the first stars born after the Big Bang and their immediate descendants should now be found preferentially near the centres (or bulges) of galaxies. Louise Howes
et al
. report observations of stars in the Milky Way bulge with some of the properties expected for these early stars formed during the 'cosmic dawn'. The authors identify more than 500 stars with an iron abundance less than 1/100th of the solar value, including one star with an iron abundance more than 10,000 times lower than solar value — without the carbon enhancement seen in stars in the outer regions of the Galaxy. Their chemical compositions are in general similar to typical halo stars of the same metallicity, although intriguing differences exist, including lower abundances of carbon.
The first stars are predicted to have formed within 200 million years after the Big Bang
1
, initiating the cosmic dawn. A true first star has not yet been discovered, although stars
2
,
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 time
5
,
6
. The Milky Way bulge underwent a rapid chemical enrichment during the first 1–2 billion years
7
, leading to a dearth of early, metal-poor stars
8
,
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.