Observation of highly excited radio recombination lines towards Cassiopeia A

Abstract

The absorption line at 26 MHz observed by Konovalenko and Sodin towards Cas A was interpreted by Blake et al. as the 631α recombination line due to carbon or heavier elements. This was confirmed subsequently by Konovalenko and Sodin, who detected the 640α line at the same optical depth as the 631α line, within error limits. These lines represent the highest excited states yet detected in interstellar gas and could provide a unique diagnostic of conditions in diffuse clouds if observed over a range of frequencies. We now report new observations of these lines carried out near 26, 38, 52 and 68 MHz using the National Radio Astronomy Observatory (NRAO) 91-m telescope at Green Bank. We detected absorption lines at all these frequencies, with a striking fourfold increase in width towards low frequencies due to collisional or radiation broadening. Assuming that these are recombination lines of carbon, the observed radial velocity corresponds well with that of the H I, OH, NH₃ and H₂CO absorption features that originate in the Perseus arm in the same direction.