A multiwavelength study of the W33 Main ultracompact HII region

Aims. The dynamics of ionized gas around the W33 Main ultracompact HII region is studied using observations of hydrogen radio recombination lines and a detailed multiwavelength characterization of the massive star-forming region W33 Main is performed. Methods. We used the Giant Meterwave Radio Telescope (GMRT) to observe the H167 alpha recombination line at 1.4 GHz at an angular resolution of 10 '', and Karl. G. Jansky Very Large Array (VLA) data acquired in the GLOSTAR survey that stacks six recombination lines from 4-8 GHz at 25 '' resolution to study the dynamics of ionized gas. We also observed the radio continuum at 1.4 GHz and 610 MHz with the GMRT and used GLOSTAR 4-8 GHz continuum data to characterize the nature of the radio emission. In addition, archival data from submillimeter to near-infrared wavelengths were used to study the dust emission and identify young stellar objects in the W33 Main star-forming region. Results. The radio recombination lines were detected at good signal to noise in the GLOSTAR data, while the H167 alpha radio recombination line was marginally detected with the GMRT. The spectral index of radio emission in the region determined from GMRT and GLOSTAR shows the emission to be thermal in the entire region. Along with W33 Main, an arc-shaped diffuse continuum source, G12.81-0.22, was detected with the GMRT data. The GLOSTAR recombination line data reveal a velocity gradient across W33 Main and G12.81-0.22. The electron temperature is found to be 6343 K and 4843 K in W33 Main and G12.81-0.22, respectively. The physical properties of the W33 Main molecular clump were derived by modeling the dust emission using data from the ATLASGAL and Hi-GAL surveys and they are consistent with the region being a relatively evolved site of massive star formation. The gas dynamics and physical properties of G12.81-0.22 are consistent with the HII region being in an evolved phase and its expansion on account of the pressure difference is slowing down.