We used the spectroscopic and astrometric data provided from the GALAH DR2 and Gaia DR2, respectively, for a large sample of stars to investigate the behaviour of the [$alpha$/Fe] abundances via two procedures, i.e. 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 $-1.3<{rm[Fe/H]}leq -0.5$ dex and age $9<tauleq13$ 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/H] 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 boundery separating the $alpha$-rich and $alpha$-poor sub-samples in the [$alpha$/Fe]$times$[Fe/H] plane.
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