How long does it take to form a molecular cloud?

Authors: Clark, Glover, Klessen & Bonnell
Link: here

The authors use a 3D hydrodynamical code (GADGET2) coupled with a chemical network (including modeling of dust and the interstellar radiation field), to self-consistently study the formation of molecular clouds at the interface between colliding flows of the warm neutral medium. It has already been shown that this formation mechanism can produce bound gas clouds in 3D simulations, but no one has yet followed the chemistry (coupled with 3D hydro) in order to establish how much of this gas could be molecular. In particular, the authors wish to establish how much CO would be produced in this scenario, since it is this which is observed, not H2.

The authors have two simulations, one fast flow (with Mach number, M=2.62) and one slow flow (with M=1.22), which both start with all gas at 5000K, 1 atm/cm^3 and solar metallicity. The fast flow case results in star formation around 10 Myr earlier than the slow flow case and there appears to be a different mechanism at work in the formation of protostellar cores (modeled by sink particles here). In the fast flow case there is a greater mass of very cold (<30K, the temperature at which we observe most CO emission), very dense (>10^4 atm/cc, the typical density of protostellar cores) gas and both H2 and CO formation occur earlier in time than in the slow flow case. However, note that the time delay between the development of H2 regions and the onset of star formation is shorter for the fast flow.

The most interesting results, however, lie in the similarities in the cloud/star formation in these two simulations. In both cases some small regions develop completely molecular hydrogen early on, and well in advance of the onset of star formation (although the exact time delay is different in the two simulations). Even more striking is the fact that, in both simulations, the fraction of CO only becomes significant 1-2 Myrs before the onset of SF i.e. the actual time delay is about the same, despite the different physical conditions. At this point there is a very rapid increase in the mass of CO, taking it quickly from undetectable to observable levels. This supports models in which there are “dark” molecular clouds; clouds containing large reservoirs of H2 that we cannot observe due to their lack of CO.