2016A&A...593L..12T

The CO⁺ reactive ion is thought to be a tracer of the boundary between a HII region and the hot molecular gas.In this study, we present the spatial distribution of the CO⁺ rotational emission toward the Mon R2 star-forming region. The CO⁺ emission presents a clumpy ring-like morphology, arising from a narrow dense layer around the HII region. We compared the CO⁺ distribution with other species present in photon-dominated regions (PDR), such as [CII] 158µm, H₂ S(3) rotational line at 9.3µm, polycyclic aromatic hydrocarbons (PAHs), and HCO⁺. We find that the CO⁺ emission is spatially coincident with the PAHs and [CII] emission. This confirms that the CO⁺ emission arises from a narrow dense layer of the HI/H₂ interface. We determined the CO⁺ fractional abundance relative to C⁺ toward three positions. The abundances range from 0.1 to 1.9x10^–10 and are in good agreement with the previous chemical model, which predicts that the production of CO⁺ in PDRs only occurs in dense regions with high UV fields. The CO⁺ linewidth is larger than those found in molecular gas tracers, and their central velocity are blueshifted with respect to the molecular gas velocity. We interpret this as a hint that the CO⁺ is probing photoevaporating clump surfaces.