Abstract
Context. Only recently, OD, the deuterated isotopolog of hydroxyl, OH, has become accessible in the interstellar medium; spectral lines from both species have been observed in the supra-Terahertz and far infrared regime. Studying variations of the OD/OH abundance amongst different types of sources can deliver key information on the formation of water, H2O. Aims. With observations of rotational lines of OD and OH towards 13 Galactic high-mass star forming regions, we aim to constrain the OD abundance and infer the deuterium fractionation of OH in their molecular envelopes. For the best studied source in our sample, G34.26+0.15, we were able to perform detailed radiative transfer modelling to investigate the OD abundance profile in its inner envelope. Methods. We used the Stratospheric Observatory for Infrared Astronomy (SOFIA) to observe the 2Π3∕2 J = 5∕2−3∕2 ground-state transition of OD at 1.3 THz (215 μm) and the rotationally excited OH line at 1.84 THz (163 μm). We also used published high-spectral-resolution SOFIA data of the OH ground-state transition at 2.51 THz (119.3 μm). Results. Absorption from the 2Π3∕2 OD J = 5∕2−3∕2 ground-state transition is prevalent in the dense clumps surrounding active sites of high-mass star formation. Our modelling suggests that part of the absorption arises from the denser inner parts, while the bulk of it as seen with SOFIA originates in the outer, cold layers of the envelope for which our constraints on the molecular abundance suggest a strong enhancement in deuterium fractionation. We find a weak negative correlation between the OD abundance and the bolometric luminosity to mass ratio, an evolutionary indicator, suggesting a slow decrease of OD abundance with time. A comparison with HDO shows a similarly high deuterium fractionation for the two species in the cold envelopes, which is of the order of 0.48% for the best studied source, G34.26+0.15. Conclusions. Our results are consistent with chemical models that favour rapid exchange reactions to form OD in the dense cold gas. Constraints on the OD/OH ratio in the inner envelope could further elucidate the water and oxygen chemistry near young high-mass stars.
Highlights
The hydroxyl radical, OH, is a key molecule to our understanding of interstellar chemistry, in particular to constrain chemical pathways leading to the formation of water, a prime molecule of interest for star- and planet formation
We find values between 3.9 × 1012 and 6.9 × 1013 cm−2, where the lowest values are typically measured towards the more evolved sources hosting ultra-compact HII (UCHII) regions, and the highest values towards the sources that are dominated by a significant amount of cold dust corresponding to quiescent regions and hot cores
Since we used the revised distance estimate of 1.4 kpc based on maser parallax measurements (Kurayama et al 2011) for G34.26+0.15, compared to Coutens et al (2014), we find that we need to scale their abundances of HDO and H128O for the outer envelope by factors of 5 and 1.5, respectively, in order to obtain a good fit to the data5
Summary
The hydroxyl radical, OH, is a key molecule to our understanding of interstellar chemistry, in particular to constrain chemical pathways leading to the formation of water, a prime molecule of interest for star- and planet formation (for a review see van Dishoeck et al 2013). Observations of rotational transitions of OH have become accessible at higher spectral resolution with the Heterodyne Instrument for the Far-Infrared aboard Herschel (HIFI; Wampfler et al 2011, 2013) and the German Receiver for Astronomy at Terahertz Frequencies (GREAT) on SOFIA (Csengeri et al 2012; Wiesemeyer et al 2012, 2016; Parise et al 2012). With this receiver, the ground-state rotational line of its deuterated form, OD, was first detected in absorption towards the envelope of the low-mass protostar IRAS 16293−2422 (Parise et al 2012)
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