2023PASJ...75...90V

We report the detection of three compact (<0.001 pc) radio sources (CRSs) at the K_a-band (0.9 cm) in the ultra-compact (UC) H II regions G040.54+2.59 (two CRSs) and G034.13+0.47 (one CRS). These CRSs have weak flux densities and are located at the center of their respective UC H II regions. We found no clear association between massive ionizing stars and CRSs but some radiative influence on the latter, as suggested by their large emission measures (>10⁷ cm^–6 pc), typical of photo-evaporating neutral objects close to or associated with massive stars. Our modelling of G40.54+2.59 shows that its CRSs supply enough ionized material to shape its morphology while significantly extending its observable lifetime. On the other hand, despite the possible relation of the CRS with the large-scale outflow signatures observed in G034.13+0.47, the influence of this CRS on the evolution of the UC H II region is unlikely. Our results show that the presence of CRSs can alleviate the so-called lifetime problem of UC H II regions. Still, to address the dynamical evolution of these regions adequately, the scenario must include additional mechanisms like ambient confinement, or the role of the kinematics of their associated stellar objects.