Water plays a crucial role both in the interstellar medium and on Earth. To constrain its formation mechanisms and its evolution through the star formation process, the determination of the water deuterium fractionation ratios is particularly suitabl
e. Previous studies derived HDO/H$_2$O ratios in the warm inner regions of low-mass protostars. We here report a detection of the D$_2$O 1$_{1,0}$-1$_{0,1}$ transition toward the low-mass protostar NGC1333 IRAS2A with the Plateau de Bure interferometer: this represents the first interferometric detection of D$_2$O - and only the second solar-type protostar for which this isotopologue is detected. Using the observations of the HDO 5$_{4,2}$-6$_{3,3}$ transition simultaneously detected and three other HDO lines previously observed, we show that the HDO line fluxes are well reproduced with a single excitation temperature of 218$pm$21 K and a source size of $sim$0.5 arcsec. The D$_2$O/HDO ratio is $sim$(1.2$pm$0.5) $times$ 10$^{-2}$, while the use of previous H$_2^{18}$O observations give an HDO/H$_2$O ratio of $sim$(1.7$pm$0.8) $times$ 10$^{-3}$, i.e. a factor of 7 lower than the D$_2$O/HDO ratio. These results contradict the predictions of current grain surface chemical models and indicate that either the surface deuteration processes are poorly understood or that both sublimation of grain mantles and water formation at high temperatures ($gtrsim$230 K) take place in the inner regions of this source. In the second scenario, the thermal desorption of the grain mantles would explain the high D$_2$O/HDO ratio, while water formation at high temperature would explain significant extra production of H$_2$O leading to a decrease of the HDO/H$_2$O ratio.
The HDO/H2O ratio is a powerful diagnostic to understand the evolution of water from the first stages of star formation to the formation of planets and comets. Our aim is to determine precisely the abundance distribution of HDO towards the low-mass p
rotostar IRAS16293-2422 and learn more about the water formation mechanisms by determining the HDO/H2O abundance ratio. A spectral survey of the source IRAS16293-2422 was carried out in the framework of the CHESS Herschel Key program with the HIFI instrument, allowing the detection of numerous HDO lines. Other transitions have been observed previously with ground-based telescopes. The spherical Monte Carlo radiative transfer code RATRAN was used to reproduce the observed line profiles of HDO by assuming an abundance jump. To determine the H2O abundance throughout the envelope, a similar study was made of the H2-18O observed lines, as the H2O main isotope lines are contaminated by the outflows. We derive an inner HDO abundance of 1.7e-7 and an outer HDO abundance of 8e-11. To reproduce the HDO absorption lines, it is necessary to add an absorbing layer in front of the envelope. It may correspond to a water-rich layer created by the photodesorption of the ices at the edges of the molecular cloud. The HDO/H2O ratio is ~1.4-5.8% in the hot corino whereas it is ~0.2-2.2% in the outer envelope. It is estimated at ~4.8% in the added absorbing layer. Although it is clearly higher than the cosmic D/H abundance, the HDO/H2O ratio remains lower than the D/H ratio derived for other deuterated molecules observed in the same source. The similarity of the ratios derived in the hot corino and in the added absorbing layer suggests that water formed before the gravitational collapse of the protostar, contrary to formaldehyde and methanol, which formed later once the CO molecules had depleted on the grains.
