NEATH – II. N2H+ as a tracer of imminent star formation in quiescent high-density gas

Author:

Priestley F D1,Clark P C1ORCID,Glover S C O2ORCID,Ragan S E1ORCID,Fehér O1,Prole L R3,Klessen R S24ORCID

Affiliation:

1. School of Physics and Astronomy, Cardiff University , Queen’s Buildings, The Parade, Cardiff CF24 3AA , UK

2. Institut für Theoretische Astrophysik, Zentrum für Astronomie, Universität Heidelberg , Albert-Ueberle-Straße 2, D-69120 Heidelberg , Germany

3. Centre for Astrophysics and Space Science Maynooth, Department of Theoretical Physics, Maynooth University , Maynooth W23 F2H6 , Ireland

4. Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Universität Heidelberg , Im Neuenheimer Feld 205, D-69120 Heidelberg , Germany

Abstract

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 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.

Funder

STFC

ERC

DFG

ERDF

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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