A study of deuterated water in the low-mass protostar IRAS16293-2422
  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 protostar 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.
