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The CO+ reactive ion is thought to be a tracer of the boundary between a HIIregion and the hot molecular gas. In this study, we present the spatialdistribution of the CO+ rotational emission toward the Mon R2 star-formingregion. The CO+ emission presents a clumpy ring-like morphology, arising from anarrow dense layer around the HII region. We compare the CO+ distribution withother species present in photon-dominated regions (PDR), such as [CII] 158 mm,H2 S(3) rotational line at 9.3 mm, polycyclic aromatic hydrocarbons (PAHs) andHCO+. 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 denselayer of the HI/H2 interface. We have determined the CO+ fractional abundance,relative to C+ toward three positions. The abundances range from 0.1 to1.9x10^(-10) and are in good agreement with previous chemical model, whichpredicts that the production of CO+ in PDRs only occurs in dense regions withhigh UV fields. The CO+ linewidth is larger than those found in molecular gastracers, and their central velocity are blue-shifted with respect to themolecular gas velocity. We interpret this as a hint that the CO+ is probingphoto-evaporating clump surfaces.

The first CO+ image: Probing the HI/H2 layer around the ultracompact HII region Mon R2