  The analysis of several spectroscopic surveys indicates the presence of a
bimodality between the disc stars in the abundance ratio space of
[${\alpha}$/Fe] versus [Fe/H]. The two stellar groups are commonly referred to
as the high-${\alpha}$ and low-${\alpha}$ sequences. Some models capable of
reproducing such a bimodality, invoke the presence of a hiatus in the star
formation history in our Galaxy, whereas other models explain the two sequences
by means of stellar migration. Our aim is to show that the existence of the gap
in the star formation rate between high-$\alpha$ and low-$\alpha$ is evident in
the stars of APOGEE DR17, if one plots [Fe/$\alpha$] versus [$\alpha$/H], thus
confirming previous suggestions by Gratton et al. (1996) and Fuhrmann (1998).
Then we try to interpret the data by means of detailed chemical models. We
compare the APOGEE DR17 red giant stars with the predictions of a detailed
chemical evolution model based on the two-infall paradigm, taking also into
account possible accretion of dwarf satellites. The APOGEE DR17 abundance
ratios [Fe/$\alpha$] versus [$\alpha$/H] exhibit a sharp increase of
[Fe/$\alpha$] at a nearly constant [$\alpha$/H] (where $\alpha$ elements
considered are Mg, Si, O) during the transition between the two disc phases.
This observation strongly supports the hypothesis that a hiatus in star
formation occurred during this evolutionary phase. Notably, the most pronounced
growth in the [Fe/$\alpha$] versus [$\alpha$/H] relation is observed for
oxygen, as this element is exclusively synthesised in core-collapse supernovae.
A chemical model predicting a stop in the star formation of a duration of
roughly 3.5 Gyr, and where the high-$\alpha$ disc starts forming from
pre-enriched gas by a previous encounter with a dwarf galaxy can well explain
the observations.
