Carbon recombination lines between 34.5 and 770 MHz toward Cassiopeia A

Abstract

We present observations of low-frequency recombination lines of carbon toward Cas A near 34.5 MHz (n ~ 575) using the Gauribidanur radio telescope and near 560 MHz (n ~ 225) and 770 MHz (n ~ 205) using the NRAO 140 foot (43 m) telescope in Greenbank. We also present high angular resolution (1') observations of the C270α line near 332 MHz using the Very Large Array in B-configuration. A high signal-to-noise ratio spectrum is obtained at 34.5 MHz, which clearly shows a Voigt profile with distinct Lorentzian wings, resulting from significant pressure and radiation broadening at such high quantum numbers. The emission lines detected near 332, 550, and 770 MHz, on the other hand, are narrow and essentially Doppler-broadened. The measured Lorentzian width at 34.5 MHz constrains the allowed combinations of radiation temperature, electron density, and electron temperature in the line-forming region. Radiation broadening at 34.5 MHz places a lower limit of 115 pc on the separation between Cas A and the line-forming clouds. Modeling the variation in the integrated line-to-continuum ratio with frequency indicates that the region is likely to be associated with the cold atomic hydrogen component of the interstellar medium, and the physical properties of this region are likely to be T_e = 75 K, n_e = 0.02 cm^-3, T_R100 = 3200 K, and n_HT_e = 10,000 cm^-3 K. Comparison of the distribution of the C270α recombination line emission across Cas A with that of ¹²CO and H I also supports the above conclusion.