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EPoS Contribution
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Molecular tracers of the threshold density for star formation
Felix Priestley Cardiff U, Cardiff, UK | |
| Star formation activity in neary molecular clouds is often found to be correlated with the amount of gas above a column density threshold of ~1022 cm-3; the reasons for the existence and value of this threshold are currently unclear. I present hydrodynamical simulations of the formation and subsequent collapse of molecular clouds coupled to a time-dependent chemical network, allowing the investigation of the cloud structure as seen in various observationally-important molecular species. I show that most traditional dense gas tracers (HCN, CS, HCO+) exist in substantial quantities in gas which is presently at high volume density (~103 cm-3), but only transiently - it eventually returns to a much more rarefied state without ever forming stars. These species provide a misleading impression of the amount of genuinely star-forming gas in a cloud. By contrast, N2H+ solely traces gas above volume densities of 104 cm-3, due to its unique chemistry. I demonstrate that forming detectable quantities of N2H+ requires gas to remain at high densities for ~0.5 Myr (roughly one free-fall time), which effectively guarantees that the gas is gravitationally bound and undergoing collapse. N2H+ thus directly traces the reservoir of material which will go on to form stars in the immediate future. I show that detectable N2H+ emission only arises when the column density exceeds the observed 1022 cm-2 threshold for star formation, and that its intensity is likely to scale linearly with the star formation rate averaged over a ~0.5 Myr timescale. | |
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| Caption: Total hydrogen and molecular column densities for simulated clouds with different initial collision velocities | |
| Collaborators: P.C. Clark, CU, UK S.C.O. Glover, ITA, DE |
Key publication
Relevant topic(s): Chemistry Collapse Molecular Clouds |
| Relevant Big Question: What determines whether gas forms stars or not? | |