RADIO RECOMBINATION LINE OBSERVATIONS OF THE GALACTIC HII COMPLEX W3

The HII complex W3 has been observed at four different frequencies (325 MHz, 1.4 GHz, 4.9 GHz and 14.7 GHz) in the radio continuum and in a number of different radio recombination lines of hydrogen, helium and carbon. From these observations the global properties of the 9 main HII components have been determined. From the hydrogen line data the electron temperature distribution could be determined for several of the compact components. Towards W3A and B locally strong helium (110-alpha and 76-alpha) lines are observed. In these components the He to H line intensity ratio is found to be greater-than-or-equal-to 20%. Locally enhancements of up to 40 +/- 5% are observed. These high He to H line intensity ratios are found in both 76-alpha VLA data and in 110-alpha WSRT data. Possible explanations for these high line ratios are (1) local He-enrichment of the ISM due to an evolved (obscured) stellar object or (2) the presence of a very hard radiation field leading to under-ionization of hydrogen. Carbon emission is detected towards the 'core' region, of W3. No carbon emission is observed at locations away from the strong continuum sources. This can only be explained if the CII cloud is located in front of these sources. Stimulated emission is found to the main emission mechanism for the C166-alpha and C110-alpha lines observed towards W3A. For this component the layer of ionized carbon is found to have a thickness approximately 0.01 pc. Narrow H 166-alpha lines are observed towards the W3 core region. The relation between the narrow hydrogen lines, the carbon lines and the molecular material is discussed. Towards W3K H272-alpha emission is observed which most likely arises in partially ionized gas associated with 'interarm' neutral hydrogen at V(lsr) approximately -25 km s-1. Towards W3E a narrow H110-alpha line is observed at V(lsr) = -28 km s-1, most likely also due to cool `interarm' gas.