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

Abstract

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 &lt; ν &lt; 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. <P />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 &lt; 1 pc) and high density (N0 &gt; 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. <P />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. <P />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.