NEATH II: N$_2$H$^+$ as a tracer of imminent star formation in quiescent high-density gas

TL;DR

This study uses simulations to show that N$_2$H$^+$ is a reliable tracer of dense, star-forming gas in molecular clouds, unlike other molecules that also appear in transient, non-star-forming regions.

Contribution

The paper demonstrates that N$_2$H$^+$ uniquely traces gravitationally bound dense gas, providing a more accurate indicator of imminent star formation compared to other molecular tracers.

Findings

N$_2$H$^+$ exists only above a volume density of 10^4 cm^{-3}.

Other molecules like HCN and HCO$^+$ are present in transient, non-star-forming gas.

N$_2$H$^+$ emission correlates with star formation activity over ~1 Myr timescales.

Abstract

Star formation activity in molecular clouds is often found to be correlated with the amount of material above a column density threshold of $\sim 10^{22} \, {\rm cm^{-2}}$. Attempts to connect this column density threshold to a ${\it volume}$ 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 $10^4 \, {\rm cm^{-3}}$, 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.