Ammonia as a Temperature Probe in Star Forming Regions Molecules in the interstellar medium have been used as probes of some of the physical and chemical conditions in the medium. The radio spectra of these molecules, which are typically caused by the rotation of the molecule and its collision with hydrogen molecules, are especially good tools for this purpose. The radio spectral region is also rich in spectral lines of more than 100 molecular species. This molecule has a rich rotation-inversion spectrum with a large number of transitions probing a wide range of excitation conditions. In molecular clouds these transitions are excited in collisions mainly with molecular hydrogen and their relative intensities reflect the kinetic temperature of the emitting medium.
Ammonia is, thus, an invaluable tool in measuring the physical conditions in molecular clouds.
Temperature determinations from NH3 have been used extensively in studies of star formation since ammonia emission has been found in regions of massive as well as low mass star formation. Such studies have proven useful in determining the conditions in molecular clouds that are necessary for star formation. These masers have been divided into two main classes - I and II and the classification is dependent on the maser transition. These masers are typically found to coincide with compact HII regions and sites of OH and H2O maser emission. Class II masers are thought to be associated with high mass star formation and appear to be radiatively pumped.