Water deuterium fractionation in star-formation regions

Abstract

Water is an essential molecule to oxygen chemistry and to the proliferation of life. Also, it is the main constituent of icy grain mantles in the Universe. The formation of water can be studied through the HDO/H₂O ratio. Thanks to the launch of the Herschel Space Observatory and the advance of sensitive sub millimeter receivers on ground-based telescopes, many H2O and HDO transitions can now be observed, enabling more accurate studies of the level of water fractionation, i.e., the enhancement of the HDO/H₂O abundance ratio over the D/H elemental ratio. Using these new technologies, we aim at revisiting the water fractionation studies toward star-forming regions. We present here detailed studies of the D/H ratios of water in one low-mass protostar and six luminous high-mass star-forming regions. Deuterated water and H₂¹⁸O has been detected in these sources with APEX, SMA and Herschel Space Observatory. We analyzed these observations using the 1D radiative transfer code RATRAN to obtain the HDO and H₂¹⁸O fractional abundances throughout the envelopes. The results show that the HDO fractional abundances in the inner and outer regions are different by more than two orders, which implies that the sublimation is very similar in low- and high-mass star-forming regions. Previous and our studies show that the values of the HDO fractional abundance in star-forming regions with different masses do not correlate with their evolution. The H₂O abundance is deduced from the analysis of the H₂¹⁸O transition lines. The similarity of the measured HDO/H₂O ratios suggests that the chemical evolution of water is the same regardless of the masses of the regions.