Molecules as Probes of the Interstellar Medium and of Star Formation

Abstract

Until the discovery of interstellar molecules at radio wavelengths in the late 1960s, the study of the interstellar medium (ISM) in its global aspect was restricted to use of the 21-cm line of HI and to optical absorption lines of several atoms (Na, Ca, Fe, K, Ti) toward nearby hot stars. To avoid excessive extinction, the optical techniques were limited both to distances of < 1 kpc from the Sun and to diffuse clouds (visual extinction < 2 magn). They provided no indication of temperatures. The 21-cm line, although visible throughout the Milky Way, also sampled gas only in diffuse regions, although over a wide range of temperatures. Indeed, prior to the use of the 21-cm line, it had already been deduced theoretically from a consideration of heating and cooling mechanisms in the ISM that the pressure versus density relation would have a minimum in pressure at a density of ~ 1 cm^-3, so that, in principle, the ISM could exist as two phases in pressure equilibrium, one being a hot (intercloud) medium, the other a cold (cloud) phase, each in thermal equilibrium at the same pressure. Nevertheless, for quite some time the ISM continued to be modeled as a relatively smoothly distributed disk of gas with a canonical temperature of 100 K and densities ranging from 0.1 cm_-3 to as high as 100 cm_-3 in extreme cases where the HI line was severely self-absorbed. Although discrete HI clouds were recognized, optical observations such as the study of the patchiness in extinction near the sun (Fitzgerald 1968) established only that the dust in clouds mujst be at least forty times as dense as in the intercloud regoins.