PDRs
Photon-dominated regions or photodissociation regions (PDRs) are those molecular clouds that are dominated by intense radiation from massive young stars. The observation of PDRs reveals the relation between star-formation and its environment:
- What is the relation between young stars, HII regions and their parental molecular clouds?
- How do spectroscopic signatures of PDRs depend on the phasical conditions such as stellar radiation field or metallicity?
- What are critical conditions for the triggering of further star formation?
Energy balance and star-formation
One of the critical parameters deciding on the fate of a dense clump is its gas temperature that governs the thermal pressure. Depending on the ratio between thermal pressure and the gravitational potential, the clump may collapse into new star(s) or disperse itself into more diffuse phases. To understand the role of triggered star formation, it is thus essential to resolve the impact of the radiation of young stars on the energy balance of PDRs.
Critical tracers
The most important cooling species in PDRs are the 159 µm [CII] and the 63 µm [OI] lines. From the simultaneous observation of both lines and the 145µm [OI] line one can determine density and temperature of the emitting region, thus resolving its energy balance (Kaufman et al. 1999, ApJ 527, 795). However, the actual determination of these parameters is always complicated by the fact that every telescope beam integrates over several regions within one line of sight with different physical conditions, including the PDR of interest, a related HII region and the diffuse interstellar medium. To disentangle the role of these different components one can exploit their different velocity structure. The components are typically seperated both by systematic velocity differences and by different velocity dispersions, i.e. line widths. It is thus essential to fully resolve all three lines with a velocity resolution of better than 1km/s to solve the puzzle of the energy balance in PDRs.
Other tracers
Today, the chemistry of PDRs is still largely unknown (Röllig et al. 2007), however, we expect that Herschel observations will lead to a large leap forward in that respect so that the role of Millimetron in that respect cannot yet be estimated.