A high HDO/H₂O ratio in the Class I protostar L1551 IRS5.

Context. Water is a very abundant molecule in star-forming regions. Its deuterium fractionation provides an important tool for understanding its formation and evolution during the star and planet formation processes. While the HDO/H2O abundance ratio has been determined toward several young Class 0 protostars and comets, the number of studies toward Class I protostars is limited. Aims. Our aim is to study the water deuteration toward the Class I protostellar binary L1551 IRS5 and to investigate the effect of evolutionary stage and environment on variations in the water D/H ratio. Methods. Observations were carried out toward L1551 IRS5 using the NOrthern Extended Millimeter Array (NOEMA) interferometer. The HDO 31, 2–22, 1 transition at 225.9 GHz and the H218O 31, 3–22, 0 transition at 203.4 GHz were covered with a spatial resolution of 0.5″× 0.8″, while the HDO 42, 2–42, 3 transition at 143.7 GHz was observed with a resolution of 2.0″ × 2.5″. We constrained the water D/H ratio using both local thermodynamic equilibrium (LTE) and non-LTE models. Results. The three transitions are detected. The line profiles display two peaks, one at ∼6 km s−1 and one at ∼9 km s−1. We derive an HDO/H2O ratio of (2.1 ± 0.8) × 10−3 for the redshifted component and a lower limit of > 0.3 × 10−3 for the blueshifted component. This lower limit is due to the blending of the blueshifted H218O component with redshifted CH3OCH3 emission. Conclusions. The HDO/H2O ratio in L1551 IRS5 is similar to the values in Class 0 isolated sources and in the disk of the Class I protostar V883 Ori, while it is significantly higher than in the previously studied clustered Class 0 sources and the comets. This result suggests that the chemistry of protostars that belong to molecular clouds with relatively low source densities, such as L1551, share more similarities with the isolated sources than the protostars of very dense clusters. If the HDO/H2O ratios in Class 0 protostars with few sources around are comparable to those found to date in isolated Class 0 objects, it would mean that there is little water reprocessing from the Class 0 to Class I protostellar stage. [ABSTRACT FROM AUTHOR]