A search for radio recombination lines in selected ultra-compact HII regions

Abstract

The role that high-mass stars play in galaxies is significant. They strongly affect their environment through their strong ultraviolet radiation, powerful winds, and supernova explosions. The physical processes that control the formation of high-mass stars are not well understood. Hence studying their early evolutionary stages, such as the formation of ultra-compact HII regions, is important in understanding their early evolution. Ultra-compact HII regions are characterized by their small sizes (d . 0:1 pc), high densities (ne & 104 cm-3), and high emission measures (EM > 107 pc cm-6). The study of these regions using observations of radio recombination line emission provides an accurate measure of their physical parameters. This is because radio waves are almost unabsorbed by the interstellar medium and can be detected from very large distances. In this study, we used the data from the Coordinated Radio and Infrared Survey for High-Mass Star Formation for Southern hemisphere (CORNISH-South), which is a high resolution, high sensitivity radio survey of the southern Galactic plane, to search for radio recombination line emissions (H87a and H112a) on 11 samples of ultra-compact HII regions. The observations were made from December 2010 to January 2012 using the Australian Telescope Compact Array (ATCA) with a receiver covering 4 - 10 GHz range. ATCA simultaneously observe each source for radio continuum and radio recombination line emission. The total integration time for each source was ~ 10 hours. The data were reduced using the Common Astronomy Software Application (CASA), developed for radio astronomy. We detected four ultracompact HII regions at 3.6 cm and nine were detected at 6 cm. No radio recombination line emission was detected towards any of the ultracompact HII regions in our sample. Hence the physical parameters were derived from the continuum observations. We found the derived physical parameters of these regions are slightly within 2o of those derived by Wood & Churchwell (1989) and Kurtz et al. (1994). Using the continuum parameters, we then derived the expected parameter for the radio recombination line emissions. We found that the line emission in these regions could possibly have widths > 25 km/s and have low intensities. This could be the main reason why we detect no radio recombination line emission.