Mapping of the nebula M17 in the H, He, and C66 alpha radio lines: Global structure and helium abundance

Results of mapping the nebula M17 in the H, He, and C66 alpha radio recombi nation lines (RRLs) are presented. The nebula was mapped using the 22-m rad io telescope: of the Lebedev Institute of Physics with an angular resolutio n of 2.6' at 11 positions cowering an area of similar to 7' x 9' (alpha x d elta). The H, He, and C56 alpha RRLs were observed toward M17S. Distributio ns of radial velocities, linewidths and electron temperatures across the ne bula were obtained; the mean electron temperature was T-e = 7770 +/- 150 K. The RRL, parameters for the northern and southern parts of the nebula diff er appreciably. The RRL profile is complex: in addition to two previously k nown components with radial velocities V-LSR similar to +20 and similar to +10 km s(-1), a third component with V-LSR similar to +46 km s(-1) has been detected. A model for the global structure of the M17 gas-dust complex is proposed, which can naturally explain the presence of the three RRL compone nts in this source. The derived distribution of ionized helium y(+) = N(He)/N(H+) across the source, together with the presence of superhot stars in M17, testifies to the fact that the HeII region at the outskirts of the rad io source extends beyond the boundary of the HII region. Using this informa tion, the multicomponent line profiles, and other measurements of y(+), the actual helium abundance is estimated to be N(He)/N(H) = 10.4 +/- 1.1%.