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

TL;DR

This study uses high-resolution ALMA data to analyze dense gas structures around the massive young stellar object W42-MME, revealing hierarchical fragmentation, complex dynamics, and signs of infall supporting a chaotic collapse scenario.

Contribution

It provides detailed multi-scale analysis of dense gas structures near W42-MME, highlighting their fragmentation, kinematics, and support for hierarchical collapse models.

Findings

19 dense gas structures identified, with transonic-supersonic motions.

Velocity dispersion correlates with structure size, indicating turbulence.

Blue asymmetry in line profiles suggests ongoing infall.

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.