HDO abundance in the envelope of the solar-type protostar IRAS 16293–2422

Abstract

Abstract. We present IRAM 30 m and JCMT observations of HDO lines towards the solar-type protostar IRAS 16293−2422. Five HDO transitions have been detected on-source, and two were unfruitfully searched for towards a bright spot of the outflow of IRAS 16293−2422. We interpret the data by means of the Ceccarelli et al. (1996) model, and derive the HDO abundance in the warm inner and cold outer parts of the envelope. The emission is well explained by a jump model, with an inner abundance x HDO in = 1 × 10 −7 and an outer abundance x HDO out ≤ 1 × 10 −9 (3σ). This result is in favor of HDO enhancement due to ice evaporation from the grains in the inner envelope. The deuteration ratio HDO/H 2 O is found to be f in = 3% and f out ≤ 0.2% (3σ) in the inner and outer envelope respectively and therefore, the fractionation also undergoes a jump in the inner part of the envelope. These results are consistent with the formation of water in the gas phase during the cold prestellar core phase and storage of the molecules on the grains, but do not explain why observations of H 2 O ices consistently derive a H 2 O ice abundance of several 10 −5 to 10 −4 , some two orders of magnitude larger than the gas phase abundance of water in the hot core around IRAS 16293−2422.

A study of deuterated water in the low-mass protostar IRAS 16293-2422

“…The deuteration fraction observed in high-mass hot cores is typically HDO/H 2 O ≤ 10 −3 (Jacq et al 1990;Gensheimer et al 1996;Helmich et al 1996), although higher values (∼10 −2 ) have recently been found for Orion (Persson et al 2007;Bergin et al 2010). This ratio has also been estimated in the inner envelope of low-mass protostars such as IRAS 16293-2422 (Parise et al 2005) at about 3%, NGC 1333-IRAS4B (Jørgensen & van Dishoeck 2010) with an upper limit of 0.06%, and NGC 1333-IRAS2A (Liu et al 2011) with a lower limit of 1%. Deuterated water has also been sought in ices towards several protostars, and has allowed Dartois et al (2003) and Parise et al (2003) to obtain upper limits of solid HDO/H 2 O from 0.5% to 2%.…”

“…More recently, Demyk et al (2010) have determined a methyl formate deuterium fractionation of ∼15%, and Bacmann et al (2010) have concluded that there is a ND/NH ratio between 30% and 100%. Singly deuterated water in IRAS 16293 has been studied with groundbased telescopes by Stark et al (2004) and Parise et al (2005). The former find a constant abundance of 3 × 10 −10 throughout the envelope with the JCMT observation of the HDO 1 0,1 −0 0,0 fundamental line at 465 GHz alone, whereas the latter obtain an inner abundance (where T ≥ 100 K) X in = 1 × 10 −7 and an outer abundance X out ≤ 1 × 10 −9 using four transitions observed with the IRAM 1 -30 m telescope, as well as a JCMT 2 observation at 465 GHz.…”

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

“…Most of the studies of water and deuterated water in star-forming regions (e.g., Ceccarelli et al 2000;Parise et al 2005;van der Tak et al 2006;Coutens et al 2012;Herpin et al 2012) assume an abundance jump at T j = 100 K, corresponding to the temperature at which the water molecules are supposed to be released in the gas phase by thermal desorption. In a first step, we consequently assumed such an abundance jump for the modeling of the HDO lines.…”

“…Deuterated water is likely to be formed through the same processes. Many rotational transitions have been detected from the ground, as well as with Herschel/HIFI, for example in low-mass protostars (Parise et al 2005;Liu et al 2011;Coutens et al 2012Persson et al 2013Persson et al , 2014, high-mass star forming regions (e.g., Jacq et al 1990;Gensheimer et al 1996), and comets (e.g., Bockelée-Morvan et al 1998;Hartogh et al 2011;Lis et al 2013). The HDO/H 2 O ratio is an interesting diagnostic tool to help understand the origin of water in the interstellar medium, with a direct comparison with the D/H ratio observed in comets and in the Earth's oceans.…”

Interferometric Studies of Low-Mass Protostars

“…Determining exactly when this ratio is established and how it varies, is an important for understanding, e.g., whether water undergoes significant processing in warm regions of protostellar envelopes or disks. Multi-transition HDO and H 2 O single-dish observations from ground-and space-based observations have previously been used to estimate the HDO/H 2 O abundance ratios in low-mass protostellar envelopes (e.g., Stark et al 2004;Parise et al 2005;Liu et al 2011). Despite the advantages offered by the ranges of excitation conditions probed by the multi-transitions observations, these studies are still complicated by the relatively large beam sizes of the ground-based single-dish and space observatories and are therefore strongly dependent on exact line radiative transfer models utilized for the interpretation.…”