The low-frequency radio recombination lines formed in the extremely extended C II regions

Abstract

Currently, nearly ten radio recombination lines are observed in the extremely extended C II regions towards Cassiopeia A, G00-00, G05-00 and G10-00. The centimetre linewidths are formed owing to the Doppler effect. The decametre linewidths are the result of combinations of the Doppler broadening, radiation interaction and electron collision transition rate. The electron collision width is determined in the impact approximation, which can be employed for highly excited atom states with the principal quantum and line numbers n in the case when 1.6×10⁵ K/n ² T _e≪1, n>500. The decametre linewidth components are tabulated together with the calculated electron densities. The dimensions of the C II regions are directly determined by using the steep dependence of the optical depth of a centimetre recombination line on the electron temperature. Solving the optical depth problem and having accounted for the spontaneous and collision transition rates in the balance equations, the highly excited atom non-equilibrium radiation lines and populations are found as functions of the principal quantum numbers in the diffusion or flux form solution approximation. The centimetre and decametre lines observed and interpreted show that the C II regions may have huge dimensions and extremely low electron densities.

Keywords:

• Low-frequency recombination line
• Highly excited carbon atom
• Extremely extended C II region

Acknowledgements

The author thanks A.A. Konovalenko, L.G. Sodin, V.M. Kontorovich and A.E. Kyrychenko for many useful comments and remarks.