Galactic H II Regions. II. Observations of Their Hydrogen 109α Recombination-Line Radiation at the Frequency 5009 MHz

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Abstract

The hydrogen lO9a recombination4ine spectrum of twenty galactic radio sources has been investigated, using a 20-channel frequency switched line radiometer (channel width 95 kHz) together with the NRAO 140-foot telescope. Sixteen of the investigated sources show line emission. The observations yield the ratio of excess line to free-free continuum temperature ( - Tc) /Tc and the radial velocity of the H ii region. The line width ApL was determined by fitting a Gaussian profile to the observed points. The kinematic distances of the H ii regions are estimated from the observed radial velocities. The distribution of the H ii regions, projected on the galactic plane, is compared with a similar analysis from optically derived distances of H ii regions in the vicinity of the Sun made by Becker and Fenkart. The rms velocity of the peculiar motion of the H ii regions is found to be less than 7 km/sec. The electron temperatures of the H ii regions are determined from the observed quantity AYL(Tc+L - Tc)/Tc, assuming thermal equilibrium. A mean value of 58000 K is found with individual values ranging from 4000 to 9500 K. Our observational results are compared with Goldberg's prediction of enhanced line radiation due to stimulated emission. Since Doppler broadening is the only effective broadening mechanism for the observed 1O9a lines, the rms velocity of the internal turbulence in the H ii regions has been determined from the observed line widths. The mean value of fifteen H ii regions yields 19.25 km/sec, a value which is more than twice the velocity of sound at Te = 5800 K. The non-detection of the 1O9a line is used to identify non-thermal sources in the Sgr A region and in the radio source associations W49 and W5 1. The velocity dispersion of H ii regions and nearby OH clouds which show the 18-cm line in emission is found to be 7.9 km/sec.

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