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

Abstract

Stars form within molecular clouds, so characterizing the physical states of molecular clouds is key to understanding the process of star formation. Cloud structure and stability are frequently assessed using metrics including the virial parameter and Larson 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 H2 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.

The 30 Doradus Molecular Cloud at 0.4 pc Resolution with the Atacama Large Millimeter/submillimeter Array: Physical Properties and the Boundedness of CO-emitting Structures

“…Correcting for this enhancement would increase log Σ lum by 0.4-1.1 (given our adopted X CO ) and bring the low column density structures shown in Figures 11 and 12 closer to virial equilibrium. We caution, however, that the virial surface density Σ vir is also affected by the underestimate of R and σ v resulting from CO-dark gas; the net effect on α vir depends sensitively on the adopted density and velocity dispersion profiles within the clumps (O'Neill et al 2022). In addition, the CO-dark gas would need to be preferentially distributed in low column density clouds, since the high column density clouds do not show an excess of apparent kinetic energy.…”

Sequential Star Formation in the Young SMC Region NGC 602: Insights from ALMA

“…where R 0 is a normalizing radius that we set to be R 0 = 0.1 pc for all clumps, and ρ c is the density of the clump at R 0 . Using the derivation in Appendix A.1 of O'Neill et al (2022), this central density can be estimated as…”