VLA observations of hydrogen and carbon recombination lines toward W3A at 1.4 GHz

Abstract

We present high-sensitivity, high-resolution VLA observations of the W3 complex of H II regions in the 168α recombination lines of hydrogen, carbon, and sulphur. The H 168α line from W3A consists of two components: a broad line (width ~27 km s^-1) and a narrow line (width ~7 km s^-1). The narrow hydrogen and carbon line emissions over W3A, although overlapping, are not entirely coextensive. The carbon line is possibly correlated with the molecular gas near W3A. Stimulated emission is the main mechanism for the narrow hydrogen line emission. The width of the H⁰ line gives an upper limit of ~1000 K for the electron temperature of the partially ionized gas. The electron density ranges from 10 to 80 cm^-3 in the narrow hydrogen line region and from 10 to 60 cm^-3 in the carbon-line region. We determined the electron temperature of the classical H II region W3A from the continuum brightness to be ~9000 K. The rms ne of this H II region is ~2200 cm^-3, and the true ne, determined from a pressure-broadened profile of the H 171η (8.6 GHz) line, is ~2 × 10⁴ cm^-3. Using these two values of electron densities, we determine a lower limit to the filling factor (0.01). Such a low value can be interpreted as an effect of density inhomogeneities in the medium.