Galactic H II Regions. III. The Nature of the Radio Source W49

The galactic radio source W49 is one of two associations of thermal sources, nonthermal sources, and OH-emission clouds found in an earlier survey of the free-free continuum and H109a recombination line radiation of twenty galactic radio sources. In this paper the nature of W49 is investigated using the results o of high angular resolution continuum observations in the frequency range 195 MHz < ν < 15 4 GHz. W49 consists of a thermal component A (G43.2+0 0) and a non-thermal component B (G43 3- 0.2). Both components are nearly at the same distance, 14.1 kpc, from the sun and at a distance of about 10 kpc from the galactic center. The projected distance between the centers of components A and B is 12.'5 or 51 pc If the non-thermal component B is a supernova remnant, it must be a considerably older and originally a much more powerful supernova remnant than Cassiopeia A. Both the analysis of the spectrum and the high angular resolution observations of the brightness temperature distribution of the radio continuum of the thermal component A suggest that it consists of several high density condensations, A2, of small size (2 R < 1 pc) and high density (N0 > 10^4 cm-3) which are imbedded in a component, Al, of lower electron density (N0 = 234 cm-3) and larger diameter (2 R = 14.4 pc). It needs from six to eleven O5 stars to ionize component A2. These high-density condensations may be the ionized shells (cocoons) of recently formed stars, whose existence has been predicted by Davidson and Harwit. Non-thermal emission of the 18-cm OH-lines close to W49 comes from two emission centers which are closely associated in space with the thermal component A. The OH- emission centers seem to coincide with the position of the thermal high-density condensations A2; this indicates a close correlation between star formation and non-thermal OH emission. The total thermal component A is surrounded by a shell of neutral hydrogen, for which an upper limit of 3.2 X 10^5 solar masses of H I is obtained. Shell and H II-regions together form a gravitationally stable system, which could be the early stage of an O- and B-star association. The non-thermal component B, probably an old supernova remnant, is located at the outer edge of the shell of neutral hydrogen.