The Pristine survey IV: approaching the Galactic metallicity floor with the discovery of an ultra-metal-poor star
Abstract
The early Universe presented a star formation environment that was almost devoid of heavy elements. The lowest metallicity stars thus provide a unique window into the earliest Galactic stages, but are exceedingly rare and difficult to find. Here, we present the discovery of an ultra-metal-poor star, Pristine_221.8781+9.7844, using narrow-band Ca H&K photometry from the Pristine survey. Follow-up medium- and high-resolution spectroscopy confirms the ultra-metal-poor nature of Pristine_221.8781+9.7844 ([Fe/H] = -4.66 ± 0.13 in 1D LTE) with an enhancement of 0.3-0.4 dex in α-elements relative to Fe, and an unusually low carbon abundance. We derive an upper limit of A(C) = 5.6, well below typical A(C) values for such ultra-metal-poor stars. This makes Pristine_221.8781+9.7844 one of the most metal-poor stars; in fact, it is very similar to the most metal-poor star known (SDSS J102915+172927). The existence of a class of ultra-metal-poor stars with low(er) carbon abundances suggest that there must have been several formation channels in the early Universe through which long-lived, low-mass stars were formed.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- December 2018
- DOI:
- 10.1093/mnras/sty2276
- arXiv:
- arXiv:1807.04292
- Bibcode:
- 2018MNRAS.481.3838S
- Keywords:
-
- stars: abundances;
- Galaxy: abundances;
- Galaxy: evolution;
- Galaxy: formation;
- Galaxy: halo;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Re-submitted to MNRAS after minor revisions