Implication of the Velocity Dispersion Scalings on High-mass Star Formation in Molecular Clouds

Abstract

This paper is aimed at exploring the implications of velocity-dispersion scalings on high-mass star formation in molecular clouds, including the scalings of Larson’s linewidth–size (σ– R) and ratio–mass surface density ( L – Σ; here L = σ/R 0.5). We have systematically analyzed the σ parameter of well-selected 221 massive clumps, complemented with published samples of other hierarchical density structures of molecular clouds over spatial scales of 0.01–10 pc. Those massive clumps are classified into four phases: quiescent, protostellar, H ii region, and PDR clumps in an evolutionary sequence. The velocity dispersion of clumps increases overall with the evolutionary sequence, reflecting enhanced stellar feedback in more evolved phases. The relations of σ– R and L – Σ are weak with the clump sample alone, but become evident when combined with others spanning a much wider spatial scales. For σ– R, its tight relation indicates a kinematic connection between hierarchical density structures, supporting theoretical models of multiscale high-mass star formation. From the L – Σ relation, cloud structures can be found to transition from overvirial state (α vir > 2) to subvirial state (α vir < 2) as they become smaller and denser, indicating a possible shift in the governing force from turbulence to gravity. This implies that the multiscale physical process of high-mass star formation hinges on the self-gravity of subvirial molecular clouds. However, the influence of turbulence may not be dismissed until large-scale clouds attain a subvirial state. This is pending confirmation from future multiscale kinematic observations of molecular clouds with uniform observing settings.