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Chemical bimodality of the Milky Way (MW) disc stars constitutes one of themost remarkable properties of MW. The cold accretion theory for thecosmological gas accretion provides one viable explanation to this phenomenon.In this scenario, the rapid cold-mode accretion in the early epoch creates thefirst generation stars relatively rich in $\alpha$-elements(O,Mg,Si,S,Ca,etc)and later cooling flow produces iron-rich second generation stars, creating thebimodality in the [$\alpha$/Fe] ratio. We employ a cosmologically motivatedchemical evolution model for disc galaxies to elucidate the role played by typeIa supernovae (SNIa), which serve as the major source of iron, in the creationof the bimodality. To this end, we divide SNIa into two groups, those formedfrom the 1st generation stars (the first SNIa) and those formed from the 2ndgeneration stars (the second SNIa). The model with the first SNIa suppressedduring the {\it second} star formation stage produces stars having high[$\alpha$/Fe] in the early phase of this stage, whereas the model whichprohibits the second SNIa produces high [$\alpha$/Fe] stars in the late phase.Both models fail to create a well-defined bimodality. We thus conclude that thecooperation of the first and the second SNIa plays a crucial role in creatingthe bimodality by maintaining rich iron content in the interstellar gasthroughout the second star formation stage.

Fundamental mechanism of the creation of chemical bimodality in the Milky Way disc in the cold accretion theory