In-situ vs accreted Milky Way globular clusters: a new classification method and implications for cluster formation

We present a new scheme for the classification of the in-situ and accreted globular clusters (GCs). The scheme uses total energy $E$ and $z$-component of the orbital angular momentum and is calibrated using [Al/Fe] abundance ratio. We demonstrate that such classification results in the GC populations with distinct spatial, kinematic, and chemical abundance distributions. The in-situ GCs are distributed within the central 10 kpc of the Galaxy in a flattened configuration aligned with the MW disc, while the accreted GCs have a wide distribution of distances and a spatial distribution close to spherical. In-situ and accreted GCs have different $[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]$ distributions with the well-known bimodality present only in the metallicity distribution of the in-situ GCs. Furthermore, the accreted and in-situ GCs are well separated in the plane of $[\mathrm{A}\mathrm{l}\mathrm{/}\mathrm{F}\mathrm{e}]-[\mathrm{M}\mathrm{g}\mathrm{/}\mathrm{F}\mathrm{e}]$ abundance ratios and follow distinct sequences in the age–$[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]$ plane. The in-situ GCs in our classification show a clear disc spin-up signature – the increase of median $V_{\phi }$ at metallicities $[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]\approx -1.3÷-1$ similar to the spin-up in the in-situ field stars. This signature signals the MW’s disc formation, which occurred $\approx 11.7-12.7$ Gyrs ago (or at $z\approx 3.1-5.3$) according to GC ages. In-situ GCs with metallicities of $[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]\gtrsim -1.3$ were thus born in the Milky Way disc, while lower metallicity in-situ GCs were born during early, turbulent, pre-disc stages of the evolution of the Galaxy and are part of its Aurora stellar component.