Transition of the initial mass function in the galaxy based on binary population synthesis

We construct a binary population synthesis model to explore the star formation history of the Galaxy. The model includes the effects of AGB evolution and binary mass transfer for a given IMF and binary period distribution function. We discuss the origins of extremely metal-poor stars with enhancement of carbon or nitrogen, with the possible effect of mass loss at low-metallicity taken into account. Our results strongly support high-mass dominated star formation during the early epoch of the Galaxy in order to explain the observed frequency of carbon and nitrogen enhancements that are thought to result from mass transfer from a former AGB binary companion. Our model also suggests that the IMF had a transition phase at [Fe/H] ∼ −2.