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

Abstract: Chemical bimodality of the Milky Way (MW) disc stars constitutes one of the most remarkable properties of MW. The cold accretion theory for the cosmological gas accretion provides one viable explanation to this phenomenon. In this scenario, the rapid cold-mode accretion in the early epoch creates the first generation stars relatively rich in $\alpha$-elements(O,Mg,Si,S,Ca,etc) and later cooling flow produces iron-rich second generation stars, creating the bimodality in the [$\alpha$/Fe] ratio. We employ a cosmologically motivated chemical evolution model for disc galaxies to elucidate the role played by type Ia supernovae (SNIa), which serve as the major source of iron, in the creation of the bimodality. To this end, we divide SNIa into two groups, those formed from the 1st generation stars (the first SNIa) and those formed from the 2nd generation stars (the second SNIa). The model with the first SNIa suppressed during the {\it second} star formation stage produces stars having high [$\alpha$/Fe] in the early phase of this stage, whereas the model which prohibits 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 the cooperation of the first and the second SNIa plays a crucial role in creating the bimodality by maintaining rich iron content in the interstellar gas throughout the second star formation stage.