Carbon recombination lines between 34.5 and 770 MHz toward Cassiopeia A

We present observations of low-frequency recombination lines of carbon towa rd Cas A near 34.5 MHz (n similar to 575) using the Gauribidanur radio tele scope and near 560 MHz (n similar to 225) and 770 MHz (n similar to 205) us ing the NRAO 140 foot (43 m) telescope in Greenbank. We also present high a ngular resolution (1') observations of the C270 alpha line near 332 MHz usi ng the Very Large Array in B-configuration. A high signal-to-noise ratio sp ectrum is obtained at 34.5 MHz, which clearly shows a Voigt profile with di stinct Lorentzian wings, resulting from significant pressure and radiation broadening at such high quantum numbers. The emission lines detected near 3 32, 550, and 770 MHz, on the other hand, are narrow and essentially Doppler -broadened. The measured Lorentzian width at 34.5 MHz constrains the allowe d combinations of radiation temperature, electron density, and electron tem perature in the line-forming region. Radiation broadening at 34.5 MHz place s a lower limit of 115 pc on the separation between Cas A and the line-form ing clouds. Modeling the variation in the integrated line-to-continuum rati o with frequency indicates that the region is likely to be associated with the cold atomic hydrogen component of the interstellar medium, and the phys ical properties of this region are likely to be T-e = 75 K, n(e) = 0.02 cm( -3), T-R100 = 3200 K, and n(H) T-e = 10,000 cm(-3) K. Comparison of the dis tribution of the C270 alpha recombination line emission across Cas A with t hat of (CO)-C-12 and H I also supports the above conclusion.