The first CO image: I. Probing the HI/H 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 compare the CO distribution with other species present in photon-dominated regions (PDR), such as [CII] 158 m, H S(3) rotational line at 9.3 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 interface. We have determined the CO fractional abundance, relative to C toward three positions. The abundances range from 0.1 to 1.9 ×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.