Collapsing Index: A New Method to Identify Star-forming Cores Based on ALMA Images

TL;DR

This paper introduces the collapsing index (CI), a new observational diagnostic derived from ALMA data, to determine whether dense cores in star-forming regions are collapsing or in hydrostatic balance, aiding understanding of star formation.

Contribution

The paper proposes a novel collapsing index (CI) based on line width radial profiles, enabling direct observational assessment of core dynamical states in star-forming regions.

Findings

Successfully applied to Orion molecular cloud data

Distinguished collapsing core from hydrostatic core observationally

Enhanced understanding of gravity-turbulence interaction in cores

Abstract

Stars form through the gravitational collapse of molecular cloud cores. Before collapsing, the cores are supported by thermal pressure and turbulent motions. A question of critical importance for the understanding of star formation is how to observationally discern whether a core has already initiated gravitational collapse or is still in hydrostatic balance. The canonical method to identify gravitational collapse is based on the observed density radial profile, which would change from a Bonnor-Ebert type toward power laws as the core collapses. In practice, due to the projection effect, the resolution limit, and other caveats, it has been difficult to directly reveal the dynamical status of cores, particularly in massive star-forming regions. We here propose a novel, straight-forward diagnostic, namely, the collapsing index (CI), which can be modeled and calculated based on the radial profile of the line width of dense gas. A meaningful measurement of CI requires spatially and spectrally resolved images of optically thin and chemically stable dense gas tracers. ALMA observations are making such data sets increasingly available for massive star-forming regions. Applying our method to one of the deepest dense-gas spectral images ever taken toward such a region, namely, the Orion molecular cloud, we detect the dynamical status of selected cores therein. We observationally distinguished a collapsing core in a massive star-forming region from a hydrostatical one. Our approach would help significantly improve our understanding of the interaction between gravity and turbulence within molecular cloud cores in the process of star formation.