Exploring the evolution of clumps in two Milky Way simulations that reproduce the alpha-abundance bimodality

In the Milky Way, there exists an alpha-abundance bimodality between young, thin disk, low-alpha stars (“low-alpha sequence”) and old, thick disk, high alpha stars (“high-alpha sequence”). While this observational characteristic of the Milky Way has been known for a long time, its origin has been unclear. Clarke et al. (2019) proposed that the alpha-bimodality is caused by clumpy star formation, where the high star formation rates in clumps in the early Milky Way caused them to self-enrich in alpha elements and create the high-alpha sequence. In contrast, the low star formation rate in the field produces the low-alpha sequence. To study the evolution of clumps more closely, we tracked the positions of individual clumps as well as the merging of clumps in two Milky Way simulations, one with 10% feedback from supernovae and the other with 20% feedback. We then used this information to study how the chemical evolution of clumps depends on properties such as mass, age, galactic radius, and star formation rate as well as their merger history. Clumps appear in the first 4 Gyr of the simulations, and are more numerous and last longer in the lower feedback simulation.