Analysis of the disc components of our Galaxy via kinematic and
  spectroscopic procedures
  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 $9<\tau\leq13$ 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.
