Radio observations of interstellar molecules

Publisher Website
Google Scholar
Abstract
Investigations of the 18-cm lines of the16O1H radical are still in an exploratory phase. The detection of the strongest line of the18O1H radical has just been reported. Searches for the microwave lines of CH and SiH are in progress.OH absorption has been observed for a few of the stronger radio sources, for a dozen galactic H 11 regions and for the source complex at the galactic centre. The ratio τOH/τHvaries by three orders of magnitude between these sources. In most cases the intensity ratios for the four OH lines are anomalous, which indicates that the populations of the energy levels are perturbed.No thermal emission from OH has been found, even where τ ≈ 1. It thus becomes exceedingly difficult to estimate excitation temperatures and to derive surface densities.Strong non-thermal emission is seen from about a third of the H 11 regions observed. The line profiles are composed of many narrow spikes, quite different for each of the four lines. The intensity ratios of the emission peaks are highly anomalous. The OH emission comes from exceedingly small areas (< 0·1 pc) on theedgeof the ionized regions, with brightness temperatures higher than 106°K.It has been suggested that the populations are inverted. Maser amplification of the background continuum then occurs. Power gains of 104to 105are required. To produce such a gain in a very small region would require high OH densities and a considerable degree of population inversion.In at least eight sources the emission is highly polarized. Complete circular polarization is common, and the sense varies rapidly between adjacent spikes in the profile. The observed polarization is incompatible with Zeeman multiplets.