Effects of CO-dark Gas on Measurements of Molecular Cloud Stability and the Size-Linewidth Relationship

TL;DR

This paper investigates how CO-dark gas influences measurements of molecular cloud stability and the size-linewidth relationship, providing correction methods and assessing their impact on understanding cloud dynamics across different environments.

Contribution

It introduces simple correction techniques for empirical cloud properties to account for CO-dark gas, clarifying its role in observed deviations from Larson's relationships.

Findings

CO-dark gas does not cause deviations from Larson's relationships in low-metallicity regions.

Virial parameters may be systematically overestimated if CO-dark gas is not accounted for.

Correcting for CO-dark gas is essential for accurate comparison of molecular cloud dynamics.

Abstract

Stars form within molecular clouds, so characterizing the physical states of molecular clouds is key in understanding the process of star formation. Cloud structure and stability is frequently assessed using metrics including the virial parameter and Larson (1981) scaling relationships between cloud radius, velocity dispersion, and surface density. Departures from the typical Galactic relationships between these quantities have been observed in low-metallicity environments. The amount of H$_2$ gas in cloud envelopes without corresponding CO emission is expected to be high under these conditions; therefore, this "CO-dark" gas could plausibly be responsible for the observed variations in cloud properties. We derive simple corrections that can be applied to empirical clump properties (mass, radius, velocity dispersion, surface density, and virial parameter) to account for CO-dark gas in clumps following power-law and Plummer mass density profiles. We find that CO-dark gas is not likely to be the cause of departures from Larson's relationships in low-metallicity regions, but that virial parameters may be systematically overestimated. We demonstrate that correcting for CO-dark gas is critical for accurately comparing the dynamical state and evolution of molecular clouds across diverse environments.