RADIO PHYSICS AND RADIO ASTRONOMY

 Subject and Purpose. In Part 2 of the current paper, we seek to analyze the observational results of decameter carbon radio recombination lines (RRLs) detected near the frequency 26 MHz through the UTR-2 radio telescope towards the S140 emission nebula and the GSH 139-03-69 super shell. These lines have proven themselves as a highly effective tool for cold, rarefied interstellar medium (ISM) diagnostics. The aim is to determine an association of line-forming regions (CII regions) with other ISM components and study physical conditions (electron temperature Te and electron density Ne) in these regions.

Methods and Methodology. By iterative comparison of detected and modeled integral intensities of decameter carbon RRLs, we determine physical state ranges where recorded experimental data best fit the model values for various combinations of T_e,N_e, and path lengths s.

Results.It has been found that the characteristics of the detected decameter carbon RRLs are consistent with the higher-frequency data for both the S140 line of sight and other Galactic plane directions, including the GSH 139-03-69 direction. Ranges of physical conditions where recorded data and model values are in the best agreement have been determined, being T_e = 50÷ 100 K, N_e = 0.01 cm^–3, and s= 10 pc — for the S140 nebula direction and its vicinity and, also, T_e = 50÷ 100 K, N_e =0.01 cm^–3, and s = 5÷ 7 pc — for the GSH 139-03-69 super shell direction.

Conclusions. The obtained results indicate that the detected decameter carbon RRLs originate from CII regions associated with clouds of diffuse neutral hydrogen HI in the Galactic plane. The lines are seen against a background Galactic radio emission whose brightness temperature increases as frequency decreases.

 Keywords: electron density; electron temperature; interstellar medium; ionized carbon; medium model; radio recombination lines; CII region

 Manuscript submitted  03.03.2023

Radio phys. radio astron. 2023, 28(4): 275-286

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