2026ApJ...996...15C

Observational studies of H II region-molecular cloud interactions constrain models of feedback and quantify its impact on the surrounding environment. A recent hypothesis proposes that a characteristic spectral signature in ground state hyperfine lines of hydroxyl (OH)-the OH flip-may trace gas that is dynamically interacting with an expanding H II region, offering a new means of probing such interactions. We explore this hypothesis using dedicated Jansky Very Large Array observations of three Galactic H II regions, G049.205-0.343, G034.256+0.145, and G024.471+0.492, in 1-2 GHz continuum emission, all four 18 cm ground-state OH lines, and multiple hydrogen radio recombination lines. A Gaussian decomposition of the molecular gas data reveals complex OH emission and absorption across our targets. We detect the OH flip toward two of our sources, G049.205-0.343 and G034.256+0.145, finding agreement between key predictions of the flip hypothesis and the observed multiwavelength spectra, kinematics, and morphology. Specifically, we demonstrate a strong spatial and kinematic association between the OH flip and the ionized gas of the H II regions-the first time this has been demonstrated for resolved sources-and evidence from ¹³CO(1-0) data that the expected OH component originates from the nondisturbed gas of the parent cloud. While we detect no flip in G024.471+0.492, we do find evidence of interacting molecular gas traced by OH, providing further support for OH's ability to trace H II region-molecular cloud interactions.