Merged H/H2 and C+/C/CO transitions in the Orion Bar
  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
CO(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 CO(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 CO(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.
