Age and kinematics of very metal-poor stars in the Galactic disk.

An investigation of 253 very metal-poor stars ([Fe/H] < -1.5) from Barklem (2005) was done to determine the age and origin of the data set. All stars are located in the Galactic halo (|b| > 20 o). To derive the age, two photometric systems were used for analysis: Johnson as the main and Gaia EDR3 as the testing one. From the isochrone fitting technique, the best fit was derived by eye-balling the star distribution in the CMD. For this purpose, two different packs of isochrones were used: Padova isochrones and a suite of Stellar Tracks and Isochrones (BaSTI). They used the most modern stellar models taking into account low metallicities. Kinematics were derived by numerical calculation of orbit in two different potentials: with and without bar. For the calculation of orbits and orbital parameters parallaxes, proper motion and radial velocities from Gaia EDR3 were employed. As a result, we found that our data set consists of three metallicity groups: Fe/H ≈ -2.0, Fe/H ≈ -2.2 - -2.5, Fe/H ≈ -3.2 with age 14 Gyr, 11 Gyr, 14 Gyr respectively (where the last result is not accurate due to the lack of stars in turnoff point region). This result is consistent with the previous derived age for individual metal-poor stars Bond (2013), VandeBerg (2014) and the age of the Universe (13.77 ± 0.06 Gyr) based on data on the cosmic microwave background (CMB), baryon acoustic oscillations, and Hubble constant (Bennett (2013), although, it tells us that these stars should be born recently after the Big Bang. The youngest population is located in the upper Galactic halo, instead, two others are coming from the lower Galactic halo. The presence of not carbon depleted Blue Strugglers is found. From a dynamical point of view, stars are separated into disk and halo stars according to their kinematics. Part of both groups is found to be affected by the bulge potential. However, stars with different metallicity present in all dynamical groups. But on average the stars following halo kinematics are more metal-poor. From chemical composition the stars under consideration show correlation between [Al/Fe] and [Fe/H] and anticorrelation for [Mg/Fe] and [Fe/H].