Phases of the ISM
Background
Today it is still not understood how molecular clouds actually form, how long they live and by which processes they are finally dispersed. It is clear that the formation is induced by a shock compression of the diffuse interstellar medium, but the critical parameters for turning material into the molecular phase are not known.
Key observations to resolve this puzzle are observations of known molecular cloud boundaries. Direct observation of the ISM parameters in the regions where the material turns molecular will help to understand the microphysics of this phase transition. Unfortunately, even the definition of such boundaries is not yet clear. Several molecules, like CH+, have been detected at conditions typical for the diffuse phase while other tracers like Co, 13CO, and SO show different locations for the transition into the molecular phase.
Thus it is essential to observe those species that govern the energy balance in the transition regions. If species are formed that provide new cooling mechanisms, they will turn the material into a thermodynamically instable regime, so that the temperature further drops, densities increase and the transition to molecular cloud conditions is triggered. Vice versa, the desctruction of these species will provide a critical threshold for the dispersal of molecular clouds.
Tracers
Main cooling species in the transition regions are C+ and OI. C+ also exists in the diffuse and ionized phase while OI should be restricted to a relatively narrow transition region at the cloud surfaces. C+ is known to be the main cooling species at low densities, while OI takes over at higher densities. By observing both species it should be possible to trace the exact cooling profiles, thus resolving the thermodynamical structure of the cloud boundaries. It is possible that the formation of just a critical amount of OI triggers the transition of the material to the molecular phase.
Essential information will come from the exact shape of the line profiles because they show the turbulent pressure in the material which is a critical parameter for the phase transition and because of the three-dimensional structure of the cloud boundaries where the velocity structure resolved in the lines may help to reduce line-of-sight confusion.
Millimetron contribution
Millimetron should observe the spatial distribution of the fine structure lines of C+ at 1.9THz and of OI at 4.7THz at about the same spatial resolution. This would allow a direct comparison of the spatial distributions of C+ and OI. By using the 12m dish for 1.9THz and the 4m dish for 4.7 THz we can resoluve the fractal structure of cloud boundaries. The spatial distribution of the pressure indicated by the line profiles gives a direct indication for the phase transition. The observations thus ask for a frequency resolution of about 2km/s or > 105 respectively,
The observations of C+ and OI need complementary data on the molecular phase, obtained in typical molecular species like CO isotopes, SO, C_2_H, which can be easily obtained from the ground and are already available for many relevant regions.