High-resolution Microwave Spectra of H and OH Absorption Lines of Cassiopeia A

Abstract

IN a recent communication Weinreb, Barrett, Meeks and Henry¹ reported the discovery of the 18-cm lines of the hydroxyl (OH) radical in the radio absorption spectra of Cassiopeia A, and the result was quickly confirmed by several others^2–4. Here we present the results of investigations of the −0.8 km/sec absorption line from Cassiopeia A at both H and OH line frequencies with a frequency resolution as narrow as 1.87 kc/s. From these observations we can derive the kinetic temperature of the absorbing gas. The equipment used in the OH observations was the same as used previously¹, except for the modification necessary to obtain the higher frequency resolution. In our earlier work a 100-kc/s portion of the spectrum was examined with a resolution of 7.5 kc/s, and in the work reported here we also examined either a 50-kc/s portion with a resolution of 3.75 kc/s or a 25-kc/s portion with a resolution of 1.87 kc/s. It is found that the − 0.8 km/sec absorption line at OH frequencies is in reality composed of two lines, separated by 7.5 kc/s, having approximately equal intensities, and having line-widths (full-width at half maximum) of 4.2 kc/s and 4.8 kc/s after correction for the receiver resolution. The observed spectra for the three frequency resolutions used are shown in Fig. 1.