The Southern Ultra-Compact HII Region Population

In this research the first high spatial resolution radio continuum survey of the southern galactic plane to date provided by the CORNISH-South survey is used to study a key stage of massive star forming regions. The CORNISH-South survey has the largest and well selected sample of UCHII regions of the southern Galactic plane. Taken with the Australia Telescope Compact Array (ATCA) at 5.5 GHz with 2.5 arcsec of resolution and 9 GHz with 1.5 arcsec of resolution in the region 295◦ <l<350◦ and|b|≤1◦.
Observations with higher resolution of the 72 UCHII regions that were not sufficiently resolved by 5.5 GHz or 9 GHz survey were carried out with the ATCA 6 km array to provide resolutions of 0.75 and 0.55 arcsec. The majority of the sources became sufficiently resolved to show morphology, and most of them appear to be cometary. Physical properties were measured and spectral indices were estimated for the higher resolution data. At 17 GHz most of the sources remain unresolved while in 22 GHz the opposite happens. 69% of the spectral indices were consistent with free-free radiation expected from UCHII regions. The sample is composed mostly of optically thin sources. 31% of the available spectral indices are found to be below the lower limit for free-free emission, which was mainly caused by underestimation of the flux densities of the extended UCHII regions at higher resolution due to filtering out of the extended emission. The new results were combined with the available data and the spectral energy distributions (SEDs) were constructed for 59 UCHII regions, which demonstrated that the 9GHz fluxes are systematically lower than the expected.
The nature of the 255 candidate UCHII regions, detected by the CORNISH-South survey at 5.5 GHz was validated analysing the measured flux densities, angular sizes and calculated 9GHz- 5GHz spectral indices. The 9 GHz observed flux densities found to be lower than expected at the SEDs, were empirically corrected, using the 5.5 to 22 GHz SEDs of the unresolved sources. The corrected fluxes were used to calculate the 9GHz-5GHz spectral indices. The UCHII regions were found be confined in the Galactic mid-plane as expected for very young massive star forming regions, fluxes were found to be in a range of 3 mJy to 2.413 Jy, and the majority of the sources are classified as resolved. 54% of the sample a found between the optically thin and optically thick limit, with most of the sources near the optically thin limit.