Of the seven different environments considered as possible sites for producing spinning dust emission by Draine and Lazarian , three are representative of the large scale diffuse interstellar medium or cirrus: the cold neutral medium (CNM), the warm neutral medium (WNM), and the warm ionized medium (WIM); five are directly associated with star formation: dark clouds, molecular clouds, the warm ionized medium, reflection nebulae, and photodissociation regions. The warm ionized medium is a cross-over component as it may be representative not only of large scale diffuse ionized plasmas, but also of hot ionized objects with compact support such as HII regions.
Photodissociation regions (PDRs), also sometimes called photon-dominated regions, contain ultraviolet photons from nearby stars, which are not energetic enough to ionize hydrogen and create HII regions, but which can dissociate most molecules.
It is a region of intermediate-mass star formation and has a high degree of photoionization towards its periphery due to UV flux from its hottest young stars, which heats and dissociates these exposed layers creating photodissociation regions (PDRs), but does not ionise the bulk of the molecular hydrogen in the cloud.
Photodissociation Regions. The first, tentative, detection of anomalous microwave emission from a PDR was made with the VSA telescope .