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 above a column density threshold of $\sim 10^{22}\mathrm{c}{\mathrm{m}}^{-2}$. 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, N${}_2$H${}^{+}$ only exists in detectable quantities above a volume density of $10^4\mathrm{c}{\mathrm{m}}^{-3}$, the point at which CO, which reacts destructively with N${}_2$H${}^{+}$, begins to deplete out of the gas phase onto 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 timescales of around a Myr.

10 pages, 10 figures. MNRAS accepted