NEATH -II. N 2 H + as a tracer of imminent star formation in quiescent high-density gas

Abstract

Star formation activity in molecular clouds is often found to be correlated with the amount of material abo v e a column density threshold of ∼10 22 cm −2. Attempts to connect this column density threshold to a volume density abo v e 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, N 2 H + only exists in detectable quantities above a volume density of 10 4 cm −3, the point at which CO, which reacts destructively with N 2 H +, begins to deplete out of the gas phase on to grain surfaces. This density threshold for detectable quantities of N 2 H + corresponds very closely to the volume density at which gas becomes irreversibly gravitationally bound in the simulations: the material traced by N 2 H + never reverts to lower densities, and quiescent regions of molecular clouds with visible N 2 H + emission are destined to eventually form stars. The N 2 H + line intensity is likely to directly correlate with the star formation rate averaged over time-scales of around a Myr.