2023MNRAS.526.4952P

Star formation activity in molecular clouds is often found to be correlated with the amount of material above a column density threshold of ${\sim} 10^{22} \, {\rm cm}^{-2}$. Attempts to connect this column density threshold to a volume density above which star formation can occur are limited by the fact that the volume density of gas is difficult to reliably measure from observations. We post-process hydrodynamical simulations of molecular clouds with a time-dependent chemical network, and investigate the connection between commonly observed molecular species and star formation activity. We find that many molecules widely assumed to specifically trace the dense, star-forming component of molecular clouds (e.g. HCN, HCO⁺, CS) actually also exist in substantial quantities in material only transiently enhanced in density, which will eventually return to a more diffuse state without forming any stars. By contrast, N₂H⁺ only exists in detectable quantities above a volume density of $10^4 \, {\rm cm}^{-3}$, the point at which CO, which reacts destructively with N₂H⁺, begins to deplete out of the gas phase on to grain surfaces. This density threshold for detectable quantities of N₂H⁺ corresponds very closely to the volume density at which gas becomes irreversibly gravitationally bound in the simulations: the material traced by N₂H⁺ never reverts to lower densities, and quiescent regions of molecular clouds with visible N₂H⁺ emission are destined to eventually form stars. The N₂H⁺ line intensity is likely to directly correlate with the star formation rate averaged over time-scales of around a Myr.