Chemical tracks using different IMFs (shown in Table 3) using the SFH derived from fitting the inner disc stars in Sect. 5 (Fig. 7, panel a)). Only the IMFs are changed, the SFH is the same in each run. The chosen IMFs are given in the legend in panel a). Panel a) shows the evolution of [Si/Fe] with stellar age. Panel b) shows the evolution of [Si/Fe] with metallicity, panel c) shows the evolution of metallicity with age, and panel d) shows the evolution of the gas fraction. The points are data from Haywood et al. (2013) and open circles depict outer thin disc stars, while solid circles are thick-disc and inner thin-disc stars.