Abstract

The interstellar medium consists of diffuse clouds and dense clouds containing atoms, molecules, and dust particles at temperatures ranging from 3 to several hundred kelvins. The dense cloud gas particles are mostly molecular and the species found vary from simple molecules such as H₂, CO, CS, HCN, etc. to heavy types such as the cyanopolyynes. Molecules other than symmetric ones, such as H₂, have dipole moments and can be observed through their rotational transitions which lie in the millimeter and submillimeter bands. The emission line strengths as seen by large ground radio telescopes are usually in the 0.1-10 K range in terms of equivalent blackbody emission. Therefore, sensitive heterodyne detectors are used where noise temperatures are only about ten times the quantum limit. The most sensitive types employ SIS detector elements, which can be used in conjunction with focal plane antenna structures such as bow-ties. Many studies have been carried out including molecule identification, temperature and density measurements, and studies of dynamics of gas clouds in our galaxy and other galaxies.

© 1986 Optical Society of America