The assembly process of star clusters: ALMA-IMF protoclusters traced by N2H+

In this talk I focus on the analysis of two of ALMA-IMF Large program protoclusters, the G353.41 and the G351.77, located at a distance of ∼2 kpc and with masses ∼2.5×10^3 M⊙ within 2pc^2 , and both embedded in larger-scale (∼8 pc) "mother filaments". We analyze the dense gas kinematics a 4 kau resolution traced by N2H+ (1−0). Focusing on G353, and from position-velocity (PV) diagrams we suggest that at large scales (~1 pc) the protocluster is undergoing slow gravitational. On small scales (~0.1 pc) we find multiple "V-shapes", or converging velocity gradients (VGs). These are consistent with gas flows toward "cores", which are small (~4 kau) condensations of mass representing the sites of formation of individual or a small number of stars. We find that V-shapes near the hub of G351 have the largest mass accretion rates in our sample (by a factor of ~77% compared to the mean), suggesting a faster collapse at the center relative to the outskirts. This indicates that the most massive stars are already forming at the center of the protocluster. Turning to G351, we find similar results and the ubiquitous presence of V-shapes in the PV diagram analysis. Hence we suggest that the conditions in G353 and G351 are generic features of Milky Way disk protoclusters. Moreover we suggest that the V-shapes may represent a useful tool as an early star formation rate tracer, although this hypothesis remains to be thoroughly tested. In more evolved protoclusters such as G012, we witness the emergence of kinematic features consistent with rotation, which are correlated with star formation indicators. In summary, I highlight the power of dense gas kinematics to "pick apart" the Milky Way cluster formation process.