PAHs and star formation

Strong IR emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 mum are a common characteristic of regions of massive star formation. These features are carried by large (similar to 50 C-atom) Polycyclic Aromatic Hydrocarbon molecules which are pumped by the strong FUV photon flux from these stars. These molecules are abundant, ubiquitous, and a dominant "force" in the interstellar medium. In particular, PAHs dominate the photoelectric heating of interstellar gas and thereby shape the phase (cloud/intercloud) structure of the ISM. PAHs are also the dominant negative charge carriers inside dense molecular cloud cores and, hence, regulate the charge balance of molecular gas. Thus, PAHs control the ambipolar diffusion process and the onset of gravitational collapse. Conversely, the spectral properties of PAHs are heavily influenced by the local physical conditions and this can be used to study the characteristics of (star formation) regions. Finally, with care, the observed strength of the PAH emission features can be used as a quantitative measure of the star formation rate.