Volume 569, September 2014
|Number of page(s)||30|
|Section||Interstellar and circumstellar matter|
|Published online||10 September 2014|
Deuteration around the ultracompact HII region Monoceros R2⋆
Instituto de Radioastronomía Milimétrica (IRAM-Spain),
Ave. Divina Pastora, 7, 20 18012
2 Los Alamos National Laboratory, PO Box 1663, Los Alamos, NM 87545, USA
3 Observatorio Astronómico Nacional, Apdo. 112, 28803 Alcalá de Henares, Madrid, Spain
4 Centro de Astrobiología (INTA-CSIC), Departamento de Astrofísica, Ctra. M-108, km. 4, 28850 Torrejón de Ardoz, Spain
5 LUTH UMR 8102, CNRS and Observatoire de Paris, place J. Janssen, 92195 Meudon Cedex, France
6 LERMA, UMR 8112, CNRS and Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, France
7 IRAM, 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
8 LERMA – LRA, UMR 8112, Observatoire de Paris and École normale Supérieure, 24 rue Lhomond, 75231 Paris, France
9 Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse, France
10 CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
11 I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
12 Department of Physics and Astronomy, UCL, Gower Place London, UK
Accepted: 29 May 2014
Context. The massive star-forming region Monoceros R2 (Mon R2) hosts the closest ultra-compact Hii region, where the photon-dominated region (PDR) between the ionized and molecular gas can be spatially resolved with current single-dish telescopes.
Aims. We aim at studying the chemistry of deuterated molecules toward Mon R2 to determine the deuterium fractions around a high-UV irradiated PDR and investigate the chemistry of these species.
Methods. We used the IRAM-30 m telescope to carry out an unbiased spectral survey toward two important positions (namely IF and MP2) in Mon R2 at 1, 2, and 3 mm. This spectral survey is the observational basis of our study of the deuteration in this massive star-forming region. Our high spectral resolution observations (~0.25–0.65 km s-1) allowed us to resolve the line profiles of the different species detected.
Results. We found a rich chemistry of deuterated species at both positions of Mon R2, with detections of C2D, DCN, DNC, DCO+, D2CO, HDCO, NH2D, and N2D+ and their corresponding hydrogenated species and rarer isotopologs. The high spectral resolution of our observations allowed us to resolve three velocity components: the component at 10 km s-1 is detected at both positions and seems associated with the layer most exposed to the UV radiation from IRS 1; the component at 12 km s-1 is found toward the IF position and seems related to the foreground molecular gas; finally, a component at 8.5 km s-1 is only detected toward the MP2 position, most likely related to a low-UV irradiated PDR. We derived the column density of the deuterated species (together with their hydrogenated counterparts), and determined the deuterium fractions as Dfrac = [XD]/[XH]. The values of Dfrac are around 0.01 for all the observed species, except for HCO+ and N2H+, which have values 10 times lower. The values found in Mon R2 are similar to those measured in the Orion Bar, and are well explained with a pseudo-time-dependent gas-phase model in which deuteration occurs mainly via ion-molecule reactions with H2D+, CH2D+ and C2HD+. Finally, the [H13CN]/[HN13C] ratio is very high (~11) for the 10 km s-1 component, which also agree with our model predictions for an age of ~0.01 to a few 0.1 Myr.
Conclusions. The deuterium chemistry is a good tool for studying the low-mass and high-mass star-forming regions. However, while low-mass star-forming regions seem well characterized with Dfrac(N2H+) or Dfrac(HCO+), a more complete chemical modeling is required to date massive star-forming regions. This is due to the higher gas temperature together with the rapid evolution of massive protostars.
Key words: astrochemistry / Hii regions / photon-dominated region (PDR) / radio lines: ISM
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2014
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