Deuteration around the ultracompact HII region Monoceros R2^⋆

¹ Instituto de Radioastronomía Milimétrica (IRAM-Spain), Ave. Divina Pastora, 7, 20 18012 Local Granada Spain
e-mail: trevino@iram.es
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

Received: 13 January 2014
Accepted: 29 May 2014

Abstract

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 D_frac = [XD]/[XH]. The values of D_frac 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 D_frac(N₂H⁺) or D_frac(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.