Modelling the chemical evolution of ω Centauri using three-dimensional hydrodynamical simulations

We present a hydrodynamical and chemical model for the globular cluster omega Centauri (omega Cen), under the assumption that it is the remnant of an ancient dwarf spheroidal galaxy (dSph), the bulk of which was disrupted and accreted by our Galaxy similar to 10 Gyr ago. We highlight the very different roles played by Type II and Type Ia supernovae (SNeII and SNeIa, respectively) in the chemical enrichment of the inner regions of the putative parent dSph. While the SNeII pollute the interstellar medium rather uniformly, the SNIa ejecta may remain confined inside dense pockets of gas as long as successive SNII explosions spread them out. Stars forming in such pockets have lower alpha-to-iron ratios than the stars forming elsewhere. Owing to the inhomogeneous pollution by SNeIa, the metal distribution of the stars in the central region differs substantially from that of the main population of the dwarf galaxy, and resembles that observed in. Cen. This inhomogeneous mixing is also responsible for a radial segregation of iron-rich stars with depleted [alpha/Fe] ratios, as observed in some dSphs. Assuming a star formation history of similar to 1.5 Gyr, our model succeeds in reproducing both the iron and the calcium distributions observed in omega Cen and the main features observed in the empirical alpha/Fe versus Fe/H plane. Finally, our model reproduces the overall spread in the colour magnitude diagram, but fails in reproducing the morphology of the anomalous subgiant branch and the double morphology of the main sequence. However, the inhomogeneous pollution reduces (but does not eliminate) the need for a significantly enhanced helium abundance to explain the anomalous position of the blue main sequence. Further models taking into account the dynamical interaction of the parent dwarf galaxy with the Milky Way and the effect of asymptotic giant branch pollution will be required.