Deuteration around the ultracompact HII region Monoceros R2

Abstract

Context. The massive star-forming region Monoceros R2 (Mon R2) hosts the closest ultra-compact Hii region, where the photondominated 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 starforming 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 C 2 D, DCN, DNC, DCO + , D 2 CO, HDCO, NH 2 D, and N 2 D + 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 N 2 H + , 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 H 2 D + , CH 2 D + and C 2 HD + . Finally, the [H 13 CN]/[HN 13 C] 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 (N 2 H + ) 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.

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde

“…The para-H 2 CO 3 22 -2 21 and 3 21 -2 20 transitions have similar upper state energies above the ground state, E u ≃ 68 K, similar spatial distributions (see Figure 2), similar line profiles (brightness temperature, linewidth, and velocity in our observations; see Figure C.1; also see Tang et al 2017a,b,c), and are often detected together in molecular clouds (e.g., Bergman et al 2011;Wang et al 2012;Lindberg & Jørgensen 2012;Ao et al 2013;Immer et al 2014;Treviño-Morales et al 2014;Ginsburg et al 2016Ginsburg et al , 2017Tang et al 2017a,b,c;Lu et al 2017). The CH 3 OH 4 22 -3 12 transition at 218.440 GHz is well separated from the para-H 2 CO (3 22 -2 21 ) transition in the OMC-1 region (see Figures 1 and C.1).…”

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde

“…In order to avoid small uncertain values in the denominator, the para-H 2 CO 3 22 -2 21 /3 03 -2 02 and 3 21 -2 20 /3 03 -2 02 ratios are most suitable to derive the kinetic temperature. The para-H 2 CO 3 22 -2 21 and 3 21 -2 20 transitions have similar energy above the ground state, E u ≃ 68 K, similar line brightness, and are often detected at the same time (e.g., Mühle et al 2007;Bergman et al 2011;Wang et al 2012;Lindberg & Jørgensen 2012;Ao et al 2013;Immer et al 2014Immer et al , 2016Treviño-Morales et al 2014;Ginsburg et al 2016;Tang et al 2016); therefore, para-H 2 CO 3 22 -2 21 /3 03 -2 02 and 3 21 -2 20 /3 03 -2 02 ratios are both good thermometers to determine the gas temperature. The kinetic temperature is traced by these two ratios with an uncertainty of 25% below 50 K ).…”

Non-thermal emission from massive star-forming regions: a possible SNR candidate G351.7–1.2?

“…If the H ii regions are triggered by this SNR, the remnant ought to be much older than the H ii regions. A typical lower limit to the age of the ultracompact H ii regions based on chemical clocks is ∼ 10 5 yr (Treviño-Morales et al 2014) and for later stages such as compact H ii regions, it is even larger. This is an order of magnitude larger than the age estimate of this SNR.…”