The first CO⁺ image

I. Probing the HI/H₂ layer around the ultracompact HII region Mon R2

¹ Instituto de Ciencia de Materiales de Madrid, Sor Juana Inés de la Cruz 3, 28049 Cantoblanco Madrid, Spain
e-mail: trevino@iram.es
² Observatorio Astronómico Nacional, Apdo. 112, 28803 Alcalá de Henares Madrid, Spain
³ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁴ CNRS; IRAP ; 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁵ LERMA, Observatoire de Paris, PSL Research University, CNRS, UMR 8112, Place Janssen, 92190 Meudon Cedex, France
⁶ Centro de Astrobiología, 28850 Torrejón de Ardoz, Spain
⁷ Université de Toulouse, UPS-OMP, IRAP, 31000 Toulouse, France
⁸ Instituto de Astrofísica de Andalucía, CSIC, 18008 Granada, Spain
⁹ Instituto de Radioastronomía Milimétrica, Ave. Divina Pastora, 7, Local 20 18012 Granada, Spain
¹⁰ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d’ Hères, France

Received: 11 May 2016
Accepted: 8 August 2016

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 compared 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 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.