Extended halos around UC HII regions in the MeerKAT GPS

The ultracompact (UC) HII region phase of a massive star overlaps significantly with its main-sequence lifetime, therefore playing an important role in improving our understanding of the birth of massive stars. The lifetime of an UC HII region is estimated to be of the order 10$^4$ years. However, previous studies have shown that the number of UC HII regions is inconsistent with their estimated lifetime. HII regions expand with time, thus their lifetimes scale directly with their size. Studies carried out on samples subject to selection biases have estimated that 80% of UC HII regions have underestimated sizes, implying a longer lifetime than 10^4 years. In this talk, I will present a large unbiased sample of HII regions drawn from the
MeerKAT Galactic Plane Survey (MeerKAT GPS). In this unbiased sample, I find that roughly a third of the UC HII regions have underestimated sizes. Therefore, while this implies a longer lifespan than the dynamical age of an UC HII region, it only provides a partial solution to the problem.