The discovery, abundances and [Fe/H]NB392 distributions of metal-poor stars in the halo of the Milky Way

By boldly looking where no one has looked before, this study surveyed a volume within the Milky Way halo to discover population III stars or their direct descendants. A cat- alogue of 123,505 sources was obtained by surveying the COSMOS field with the WFC and the NB392 filter (λc=3918 ̊A, ∆λ=52 ̊A), sensitive to CaHK. A star-galaxy cut was devised from samples of confirmed stars and galaxies based on spectroscopic redshifts and the apparent optical and infra-red colours of the samples, with a completeness and contamination of 75.3% and 6.1% respectively. The catalogue was reduced to 338 potential stars by introducing a limiting magnitude of 24.7, making it 5-7 magni- tudes deeper than the Pristine survey and corresponding to a limiting distance of 166 kpc for G-type stars. This survey covered 1.89 deg2, corresponding to volumes from (1.89 × 108)pc3 to (8.55 × 1016)pc3 from spectral types M to O. Stars with known metallicity, [Fe/H], values from the POLLUX database were used to create an [Fe/H] (NB392−u) − (g − i) by (g − i) grid and the 338 candidates were assigned metallicity values. This created a final catalogue that was sorted by metallicity and candidates with [Fe/H]<−3 were visually inspected using the HST F814W filter to determine whether they were stars, in which seven were found. These could give an insight to the earliest stars in the universe. Number densities were calculated for F, G and K-type stars with [Fe/H] values of −4 to be (4.3±3.0)×10−14 pc −3, (1.36±0.96)×10−13 pc −3 and (3.0 ± 2.1) × 10−11 pc −3 respectively. It was found that the number density of stars with [Fe/H]<−3 is less than expected by a factor of ∼ 10.8, however it is ≈ 6.3 times larger for stars with [Fe/H]<−4