⁵⁶Ni mass synthesized during the SN Ia explosion as a function of the progenitor iron abundance ((Fe/H)_progenitor) and α-element enrichment ((α/Fe)_progenitor). The TBT03 model is plotted as a solid black line, even in the expression of Z_progenitor/Z_⊙. The solid red lines show the average of 10 000 linear regression results (light red lines) from the LINMIX package. In the left panel, dot-dashed and dotted purple lines present the TBT03 model, altered for thick- and thin-disk models by H09, respectively. The vertical dashed line in the right panel indicates (α/Fe)_progenitor = 0.35, showing a clear gap as discussed by K24. The green squares represent the weighted-means of ⁵⁶Ni mass in each (α/Fe)_progenitor bin.