(Re)mind the gap: A hiatus in star formation history unveiled by APOGEE DR17

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 confirming previous suggestions. We then 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 into account also the possible accretion of dwarf satellites. The APOGEE DR17 abundance ratios Fe/alpha versus alpha /H exhibit a sharp increase in 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. The revised version of the two-infall chemical evolution model proposed in this study reproduces the APOGEE DR17 abundance ratios better than before. Particularly noteworthy is the model's ability to predict the hiatus in the star formation between the two infalls of gas, which form the thick and thin disc, respectively, and thus generate abundance ratios compatible with APOGEE DR17 data. We show that the signature of a hiatus in the star formation is imprinted in the APOGEE DR17 abundance ratios. A chemical model predicting a pause in the star formation of a duration of roughly 3.5 Gyr, and in which the high-alpha disc starts forming from pre-enriched gas by a previous encounter with a dwarf galaxy, could well explain the observations