Abstract
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 compare the CO(+) distribution with other species present in photon-dominated regions (PDR), such as [CII] 158 µm, H(2) S(3) rotational line at 9.3 µm, 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/H(2) interface. We have determined the CO(+) fractional abundance, relative to C(+) toward three positions. The abundances range from 0.1 to 1.9 ×10(-10) and are in good agreement with 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 blue-shifted with respect to the molecular gas velocity. We interpret this as a hint that the CO(+) is probing photo-evaporating clump surfaces.