Gas Kinematics of the Massive Protocluster G286.21+0.17 Revealed by ALMA

Abstract: We study the gas kinematics and dynamics of the massive protocluster G286.21+0.17 with the Atacama Large Millimeter/submillimeter Array using spectral lines of $C^{18}O$(2-1), $N_2D^+$(3-2), $DCO^+$(3-2) and $DCN$(3-2). On the parsec clump scale, $C^{18}O$ emission appears highly filamentary around the systemic velocity. $N_2D^+$ and $DCO^+$ are more closely associated with the dust continuum. $DCN$ is strongly concentrated towards the protocluster center, where no or only weak detection is seen for $N_2D^+$ and $DCO^+$, possibly due to this region being at a relatively evolved evolutionary stage. Spectra of 76 continuum defined dense cores, typically a few 1000 AU in size, are analysed to measure their centroid velocities and internal velocity dispersions. There are no statistically significant velocity offsets of the cores among the different dense gas tracers. Furthermore, the majority (71\%) of the dense cores have subthermal velocity offsets with respect to their surrounding, lower density $C^{18}O$ emitting gas. Within the uncertainties, the dense cores in G286 show internal kinematics that are consistent with being in virial equilibrium. On clumps scales, the core to core velocity dispersion is also similar to that required for virial equilibrium in the protocluster potential. However, the distribution in velocity of the cores is largely composed of two spatially distinct groups, which indicates that the dense molecular gas has not yet relaxed to virial equilibrium, perhaps due to there being recent/continuous infall into the system.