The ultracompact regions G40.54+2.59 and G34.13+0.47: A new detection of compact radio sources

Abstract

We report the detection of three compact ($<0.001$ pc) radio sources (CRSs) at K${}_a$-band (0.9 cm) in the \uchiirs 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 \uchii 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 (> $10^7{\mathrm{cm}}^{-6}\mathrm{pc}$), typical of photo evaporating neutral objects close to or associated with massive stars. Our modelling of G40.54+2.59 shows that their 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 \uchii region is unlikely. Our results show that the presence of CRSs can alleviate the so-called lifetime problem of UCHII regions. Still, to address their dynamical evolution adequately, the scenario must include additional mechanisms like ambient confinement, or the role of the kinematics of their associated stellar objects.

Published in Publications of the Astronomical Society of Japan, Volume 75, Issue 1, February 2023, Pages 90-102, https://doi.org/10.1093/pasj/psac090

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