biostudies-literatureUnknown593Trevino-Morales SPThe 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, H2 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/H2 interface. We have determined the CO+ fractional abundance, relative to C+ toward tAstronomy and astrophysicsL12https://www.ebi.ac.uk/biostudies/studies/S-EPMC5055094biostudies-literatureThe first CO+ image: I. Probing the HI/H2 layer around the ultracompact HII region Mon R2.PMC5055094Trevino-Morales SPOssenkopf-Okada VGonzalez-Garcia MFuente ACernicharo JRoueff EGerin MPety JSanchez-Monge AGarcia-Burillo SKramer CRizzo JRBerne OPilleri PGoicoechea JRfalseThe 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 compare the CO+ distribution with other species present in photon-dominated regions (PDR), such as [CII] 158 µm, H2 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/H2 interface. We have determined the CO+ fractional abundance, relative to C+ toward t2016-01-01T00:00:00Z2016 Sep2025-04-04T18:39:24.885Z2019-03-27T02:26:10ZS-EPMC50550942772151510.1051/0004-6361/201628899