tokens: iron. Finally, the microturbulent velocity (ξ t ) is established by erasing any trend between the abundance from Fe i and EWr. The same approach to constrain ξ t is used consistently in the three independent analysis. The atmospheric parameters are then set in an iterative fashion, first setting the temperature, and then revisiting T eff as the gravity and microturbulence are tweaked. The error associated with the determination of ξ t is estimated by propagating the uncertainty in the slope in the rEW versus Fe abundance plane. This uncertainty is of the order of ≃0.07 km s −1 . The derived T eff are typically based on more than ≃70–130 Fe i lines and have internal uncertainties of about 50–60 K, while the internal uncertainties in log g are of the order of ≃0.06 dex. The derived atmospheric parameters and [Fe i /H] and [Fe ii /H] abundances are listed in Table 2 , along with their associated uncertainties. The left-hand panel of Fig. 2 illustrates how they compare to Y14 results. We observe that our T eff , log g , and ξ t estimates are in excellent agreement with those listed in Y14 , being the average difference between our and Y14 estimates Δ T eff = −19 ± 8 K, Δlog g = −0.0 ± 0.0 dex, Δξ t = −0.09 ± 0.02 km s −1 . The agreement between our [Fe i /H] and [Fe ii /H] abundances and those listed in Y14 is also very good. In particular, we note that the metal-poor stars identified by Y14 , i.e. see Table 1 , are also the most metal-poor stars in our analysis (see Table 2 ). Additionally, the four stars with [Fe/H] ≃ −1.0 dex are the same metal-rich stars in Y14 . In the left-hand panel of Fig. 3 , we plotted the histograms of the [Fe i /H] and [Fe ii /H] distributions when the full spectroscopic analysis is performed. Our ability to portrait the [Fe i /H] and [Fe ii /H] distributions for this small data set critically depends on the adopted bin size (Δ). To adequately represent the shape of the underlying distribution, in Fig. 3 we selected Δ as the value which minimize C (Δ) = (2 me − va )/Δ 2 ; where me and va are the mean and variance of the data set. As one can see, we obtain an iron dispersion that mirrors the dispersion measured by Y14 . We find a large (Δ[Fe/H] ≃ 0.8 dex) iron distribution with two prominent peaks at [Fe/H] ≃ −1.7 and −1.0 dex and a smaller component at [Fe/H] ≃ −1.5 dex. The iron dispersion within the metal-poor stars is larger (Δ[Fe/H] ≃ 0.4 dex) than that expected from observational errors alone, indicating the presence of two separated subpopulations with different metallicity. Photometric analysis . We use VI WFI photometry to derive photometric T eff and log g * estimates. 2 Input T eff values have been computed by means of the ( V − I ) 0 – T eff transformation by Alonso, Arribas Martínez-Roger ( 1999 ) adopting a colour excess E ( B − V ) = 0.02 mag taken from the most update version (2010) of the McMaster catalogue (Harris 1996 ). Surface gravities have been computed assuming the photometric T eff , the
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