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

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] 158 μ m, H 2 S(3) rotational line at 9.3 μ m, 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 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 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.