PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF AUSTRALIA, vol.36, 2019 (SCI-Expanded)
We used the spectroscopic and astrometric data provided from the GALactic Archaeology with HERMES (GALAH) Data Release (DR2) and Gaia DR2, respectively, for a large sample of stars to investigate the behaviour of the [alpha/Fe] abundances via two procedures, that is, kinematically and spectroscopically. With the kinematical procedure, we investigated the distribution of the [alpha/Fe] abundances into the high-/low-probability thin disc, and high-/low-probability thick-disc populations in terms of total space velocity, [Fe/H] abundance, and age. The high-probability thin-disc stars dominate in all sub-intervals of [alpha/Fe], including the rich ones: [alpha/Fe] > 0.3 dex, where the high probability thick -disc stars are expected to dominate. This result can be explained by the limiting apparent magnitude of the GALAH DR2 (V < 14 mag) and intermediate galactic latitude of the star sample. Stars in the four populations share equivalent [alpha/Fe] and [Fe/H] abundances, total space velocities, and ages. Hence, none of these parameters can be used alone for separation of a sample of stars into different populations. High-probability thin-disc stars with abundance -1.3 < [Fe/H] <= -0.5 dex and age 9 < tau <= 13 Gyr are assumed to have different birth places relative to the metal -rich and younger ones. With the spectroscopic procedure, we separated the sample stars into alpha-rich and alpha-poor categories by means of their ages as well as their [alpha/Fe] and [Fe/11] abundances. Stars older than 8 Gyr are richer in [alpha/Fe] than the younger ones. We could estimate the abundance [alpha/Fe] = 0.14 dex as the boundary separating the alpha-rich and alpha-poor sub-samples in the [alpha/Fe] x [Fe/H] plane.