In this work, we analyze a sample of 28 extremely metal-poor field stars in the solar vicinity and determine their ages. To this aim, we use Gaia data to derive their distances. Moreover, we employed different reddenings and superimpose isochrones from different sources in the star’s color-magnitude diagram built up with different photometric systems. We highlight subtle metallicity effects when using the Johnson photometry for low metallicity stars and finally adopt Gaia photometry. An automatic fitting method is devised to assign ages to each individual star taking into account the uncertainties. The mean age of the sample is 13.9 ± 0.5 Gyr using Padova isochrones, and 13.7 ± 0.4 Gyr using BASTI isochrones. We found also a group of very metal-poor stars ([Fe/H]: -2.7 – -2.0 dex) with relatively young ages: 8 – 10 Gyr. Also, we investigated the chemical and kinematic properties of these stars with the goal of obtaining some insights into their origin and their parent population. We did not find any Al-Mg anticorrelation, which shows that these stars did not form in globular clusters, while the detailed analysis of their orbital parameters reveals that these stars are most probably associated with the pristine bulge of the Milky Way. We then sketch a scenario for the formation of the Milky Way in which the first structure to form was the bulge through rapid collapse. The other components have grown later on, with a significant contribution of accreted structures.
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