The Scutum ring of HII regions, G24.6 + 0.0, associated with CO and HI shells

Radio observations of the ring complex G24.6 + 0.0 or the Scutum ring were made at 10, 5, and 2.7 GHz in continuum and at 115.27 GHz in the CO(J = 1 - 0) line emission. The complex is composed of several compact HII regions associated with a diffuse extended radio emission centered on l = 24.6, b = 0.0. The spectrum of the diffuse component is flat, indicating its thermal origin. According to the radial velocities of the compact HII regions they are located in the Scutum arm at 9.1 kpc from the sun, composing a shell-like association of radius 35 pc embedded in an extended HII region. CO observations have revealed association of an arcshaped molecular cloud of mass 3 x 105 solar mass aligned concentric to the HII ring having a sharp inner edge, most likely being shock-compressed from inside. This molecular arc is further surrounded by a larger arc of HI gas of radius 70 pc and a mass of 2 x 104 solar mass. The positional relationship of the HII, H2(CO), and HI gases suggests a typical sequential star formation on a shock-compressed CO shell being sandwitched between an expanding HII region and the larger HI cloud. (author)