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 < { [ Fe / H ] } \leq - 0.5 dex and age 9 < \tau \leq 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/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 .