Volume 593, September 2016
|Number of page(s)||6|
|Published online||15 September 2016|
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
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
Accepted: 8 August 2016
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.
Key words: astrochemistry / photon-dominated region / Hii regions / radio lines: ISM
© ESO, 2016
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