Identification of the anomalous 26.131-MHz nitrogen line observed towards Cas A

Abstract

Konovalenko and Sodin1 recently reported the detection of an absorption line at a frequency of 26.131 MHz towards Cassiopeia A and attributed the line to the 26.127 MHz F = 5/2 → 3/2 hyperfine transition of 14N. The difference in frequency corresponds to a radial velocity of –40 km s–1, which is compatible with the mean velocity of the Perseus arm H I absorption lines seen towards Cas A. However, to explain the strength of the line, Konovalenko and Sodin require (N/H) = 8 × 10–3 compared with the cosmic ratio of 1.2 × 10–4. An anomalous abundance of this magnitude would have substantial impact on ideas about galactic nucleosynthesis. We suggest here that the line is a 631α recombination line due to neutral atoms of heavy elements—either carbon atoms in the H I clouds or possibly heavier atoms (Mg, Ca, Fe) in hot (∼104 K) ionized gas along the line of sight to Cas A.