CHARACTERISATION OF CORNISH-SOUTH ULTRA-COMPACT HII-REGIONS SAMPLE USING THE MeerKAT

Abstract

Ultra-compact HII (UC HII) regions are small and dense ionized regions surrounding massive protostars. They are formed when ultra-violet (UV) radiation from massive protostars ionizes the surrounding neutral hydrogen gas. UC HII regions represent the earliest stages of massive star formation; hence it is important to characterise them for a better understanding of the process of massive star formation. However, the mystery of the birth and early life of massive stars is yet to be solved due to some challenges such as their short lifetimes, rarity, and large distances, in constructing a complete evolutionary sequence. In addition, the massive protostars are born in a dusty environment which results in high extinction caused by dust grains making it challenging to observe the embedded protostars and their immediate environment in optical and infrared wavelengths. This limitation can be partially overcome by using radio observations that is less affected by dust extinction. Thus, the advantage of UC HII regions in studying massive star formation is that they emit strong radio continuum radiation due to the presence of free electrons within the ionized gas, enabling their detection even at large distances by radio telescopes. In this study, the radio data from the MeerKAT (1.3GHz) and CORNISH-South (5.5 GHz) surveys were used to conduct a comprehensive characterization of UC HII regions. The MeerKAT survey provided the opportunity to study these embedded objects at lower frequencies, where dust extinction has a minimal effect. With this data, I was able to estimate the radio spectral indices (𝛼) of 79 samples of ultra-compact HII regions. It was found that 91% of these sources had 𝛼 ranging from −0.1 to 2 between 1.3 GHz and 5.5 GHz thus indicating that they are thermal. The remaining 9% of the UC HII regions had 𝛼 less than −0.1 at radio frequency, which may be interpreted as the presence of non-thermal emission. However, this effect is caused by the fact that the MeerKAT observation is more sensitive to extended emission than the CORNISH-South observation. Further characterisation of these sources involved the determination of brightness temperature at 5.5 GHz (ranges from ~100.5K to ~104K), optical depth (range: 0.0 to 0.1), Lyman continuum fluxes ( peaks between 46.5 < (logNi) < 48.6 photons 𝑠 −1 ), emission measure (range from 106 to 108 pc cm-6 ), electron densities (peaks between 103 and 104 electrons cm−3 ), physical size (ranging from 0.01 pc to 0.05 pc) which were all consistent with UC HII regions. Finally, investigation for the presence of protostars in the UC HII regions sample was investigated by computing their radio luminosities and comparing them with those of protostars. It was found that the UC HII regions sample was not contaminated with any massive protostellar objects (MYSO jets and low-mass stars)