Water deuterium fractionation in the high-mass star-forming region G34.26+0.15 based on Herschel/HIFI data

Abstract

Understanding water deuterium fractionation is important for constraining the mechanisms of water formation in interstellar clouds. Observations of HDO and H 18 2 O transitions were carried out towards the high-mass star-forming region G34.26+0.15 with the HIFI instrument onboard the Herschel Space Observatory, as well as with ground-based single-dish telescopes. Ten HDO lines and three H 18 2 O lines covering a broad range of upper energy levels (22-204 K) were detected. We used a non-LTE 1D analysis to determine the HDO/H 2 O ratio as a function of radius in the envelope. Models with different water abundance distributions were considered in order to reproduce the observed line profiles. The HDO/H 2 O ratio is found to be lower in the hot core (∼3.5 × 10 −4 -7.5 × 10 −4 ) than in the colder envelope (∼1.0 × 10 −3 -2.2 × 10 −3 ). This is the first time that a radial variation of the HDO/H 2 O ratio has been found to occur in a high-mass source. The chemical evolution of this source was modeled as a function of its radius and the observations are relatively well reproduced. The comparison between the chemical model and the observations leads to an age of ∼10 5 years after the infrared dark cloud stage.

On the accretion process in a high-mass star forming region

“…A cloud-cloud collision in our line of sight between two components with two different density profiles, moving inwards. Alternatively one cloud moving inwards and the outer outwards, which would be in agreement with what Coutens et al (2014) found in the inner region from HDO observations and modeling. 4.…”

“…Comparing our low derived abundances with the chemical models for a relatively dense gas (n H 2 = 2 × 10 5 cm −3 , T = 16 K) presented in Coutens et al (2014) suggests that the molecular gas in G34 is not in steady-state, with an age of only about 10 5 years. This can be the result of the short evolutionary timescales of massive stars.…”

“…-Micro turbulence: (i) a constant turb profile is set through the whole cloud and varied between 0.5 and 2 km s −1 ; (ii) a step profile with a constant inner and outer turb is varied together with the boundary radius, as free parameters. The profile in the outer envelope is set to a higher value compared to the inner, as measured by Liu et al (2013), Coutens et al (2014) for G34 and by Herpin et al (2012), Caselli & Myers (1995) for other massive protostars.…”

“…6.1. The models used to describe G34 by Wyrowski et al (2012) and Coutens et al (2014) are discussed and compared with the best-fit model in Sect. 6.2.…”

Water deuteration and ortho-to-para nuclear spin ratio of H2in molecular clouds formed via the accumulation of H I gas

“…When the gas temperature exceeds 100 K, we use the reaction sets in Harada et al (2010Harada et al ( , 2012 instead of those in Garrod & Herbst (2006) for the neutral-neutral reactions and neutral-ion reactions; the network of Harada et al (2010Harada et al ( , 2012 was designed for modeling hightemperature gas-phase chemistry (100−800 K) and includes reactions with a high potential energy barrier. Our network has been extended to include mono, doubly, and triply deuterated species Furuya et al 2013) and nuclear spin states of H 2 , H + 3 , and their isotopologues (Hincelin et al 2014;Coutens et al 2014b Hugo et al (2009). In the rest of this sections, we describe the treatment of interaction between gas and (icy) grain surface, surface reactions (Sect.…”