Aperture synthesis observations of galactic H II regions. II. The galactic radio source W58.

Summary. The strong gaJactic radio source W 58, containing the H ii regions S 99, S 100, NGC 6857, the peculiar object K 3-50 and the OH maser source ON-3 has been observed with the Westerbork Synthesis Radio Telescope at wavelengths of 49, 21 and 6 cm. The results, combined with previous work, indicate: i) W 58 consists of several H ii regions with widely varying physical parameters. These H ii regions form a giant physical complex together with large amounts of neutral hydrogen and dust. The present appearance of the complex is best explained in terms of a very rich and very massive OB association, or group of OB associations coming into being. ii) From the presence of large (d =30 pc) and low- density (fle = 1015 cm -3) diffuse emission regions one deduces that the process of star formation started at the near edge of the distant (D =9 kpc) complex. Since then, the region of star formation appears to have moved into the complex over a distance of a few parsecs. At present, at least a dozen early 0 stars is needed to explain the observed radio luminosity and structure of the H ii regions in the complex. iii) K 3-50 is a multiple radio source. It consists of a compact component A 2 (d=0.2 pc, e= 1 x t0 ) surrounded by a halo A3 (d=0.8pc, e= 1.5 x ). A 2 is the component associated with the optical object K 3-50. It has a turnover frequency of 2700 MHz. The near infrared emission at wavelengths of 2 m and larger originates in a second component A 1 (d = 0.1 pc, e =5 x 10 cm -3) that is heavily obscured (Ay SO ), and that has a turnover frequency of 8500 MHz. iv) Source C associated with ON-3 is also multiple. The compact source C 1 (d=0.1 pc, fle=2X t0 ) coincides with ON-3 and an H2O source and probably is surrounded by a dense cocoon. Two more compact components C 2 and C 3 are slightly larger (d =0.2 pc, =6 x 10 cm -3) and are associated with an envelope (d=1.6pc,ne=7x ). v) An optically obscured (Ay> 10 ) source and an optically visible source (NGC 6857, Av = 4W5) both show an uneven radio brightness distribution with a central minimum. The exciting star of NGC 6857 is located in the hole. Both sources have a diameter in the order of 1.5 pe and r.m.s. electron-densities of the order of (2 to 3)x t0 vi) Several more evolyed H ii regions, most of them visible, have r.m.s. electron-densities around 500 cm -3 Among them are S99 and S 100. The nature of one compact source (H) is unce?tain. vii) W 58 as a whole appears to suffer 4W5 of extinction due to foreground material. If the distance is 9 kpc this corresponds to 0W5 of extinction per kiloparsec. Key words: H ii regions - star formation - K 3-50 - W 58 - galactic radio source