Observation of highly excited radio recombination lines towards Cassiopeia A

The authors report new observations of highly excited radio recombination lines towards Cassiopeia A, carried out near 26, 38, 52 and 68 MHz using the National Radio Astronomy Observatory 91-m telescope. Absorption lines were detected 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 HI, OH, NH3 and H2CO absorption features that originate in the Perseus arm in the same direction. (author)