Mapping of the nebula M17 in the H, He, and C66α radio lines: Global structure and helium abundance

Details Mapping of the nebula M17 in the H, He, and C66α radio lines: Global structure and helium abundance Mapping of the nebula M17 in the H, He, and C66α radio lines: Global structure and helium abundance

Aug 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999arep...43..511t&link_type=abstract

Astronomy Reports, Volume 43, Issue 8, August 1999, pp.511-520

Astronomy and Astrophysics

Astronomy

5

Scientific paper

Results of mapping the nebula M17 in the H, He, and C66alpha radio recombination lines (RRLs) are presented. The nebula was mapped using the 22-m radio telescope of the Lebedev Institute of Physics with an angular resolution of 2.6' at 11 positions covering an area of ~ 7' x 9' (alpha x delta). The H, He, and C56alpha RRLs were observed toward M17S. Distributions of radial velocities, linewidths and electron temperatures across the nebula were obtained; the mean electron temperature was T_e = 7770 +/- 150 K. The RRL parameters for the northern and southern parts of the nebula differ appreciably. The RRL profile is complex: in addition to two previously known components with radial velocities V_LSR ~ +20 and ~ +10 km s^-1, a third component with V_LSR ~ +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 components 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 radio source extends beyond the boundary of the HII region. Using this information, 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%.

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