2019MNRAS.486.2525K

High-resolution ALMA images towards the Orion Bar show no discernible offset between the peak of H₂ emission in the photodissociation region (PDR) and the ¹³CO(3-2) and HCO⁺(4-3) emission in the molecular region. This implies that positions of H₂ and CO dissociation fronts are indistinguishable in the limit of ALMA resolution. We use the chemo-dynamical model MARION to show that the ALMA view of the Orion Bar, namely, no appreciable offset between the ¹³CO(3-2) and HCO⁺(4-3) peaks, merged H₂ and CO dissociation fronts, and high intensity of HCO⁺(4-3) emission, can only be explained by the ongoing propagation of the dissociation fronts through the molecular cloud, coupled to the dust motion driven by the stellar radiation pressure, and are not reproduced in the model where the dissociation fronts are assumed to be stationary. Modelling line intensities, we demonstrate that after the fronts have merged, the angular separation of the ¹³CO(3-2) and HCO⁺(4-3) peaks is indeed unresolvable with the ALMA observations. Our model predictions are consistent with the results of the ALMA observations about the relation of the bright HCO⁺(4-3) emission to the compressed gas at the border of the PDR in the sense that the theoretical HCO⁺(4-3) peak does correspond to the gas density enhancement, which naturally appears in the dynamical simulation, and is located near the H₂ dissociation front at the illuminated side of the CO dissociation front.