Fragmentation and dynamics of dense gas structures in the proximity of massive young stellar object W42-MME

Abstract: We present an analysis of the dense gas structures in the immediate surroundings of the massive young stellar object (MYSO) W42-MME, using the high-resolution (0$''$.31$\times$0$''$.25) ALMA dust continuum and molecular line data. We performed a dendrogram analysis of H$^{13}$CO$^{+}$ (4-3) line data to study multi-scale structures and their spatio-kinematic properties, and analyzed the fragmentation and dynamics of dense structures down to $\sim$2000 AU scale. Our results reveal 19 dense gas structures, out of which 12 are leaves and 7 are branches in dendrogram terminology. These structures exhibit transonic-supersonic gas motions (1$<\mathcal{M}<5$) with overvirial states ($\alpha_{\rm vir}\geq2$). The non-thermal velocity dispersion-size relation ($\sigma_{\rm nt}-L$) of dendrogram structures shows a weak negative correlation, while the velocity dispersion across the sky ($\delta\mathit{V_{\rm lsr}}$) correlates positively with structure size ($L$). Velocity structure function ($S_{2}(l)^{1/2}$) analysis of H$^{13}$CO$^{+}$ data reveals strong power-law dependencies with lag ($l$) up to a scale length of $\lesssim$ 6000 AU. The mass-size ($M-R$) relation of dendrogram structures shows a positive correlation with power-law index of 1.73$\pm$0.23, and the leaf L17 hosting W42-MME meets the mass-size conditions for massive star formation. Blue asymmetry is observed in the H$^{12}$CO$^{+}$ (4-3) line profiles of most of the leaves, indicating infall. Overall, our results observationally support the hierarchical and chaotic collapse scenario in the proximity of the MYSO W42-MME.