  Although deuterium enrichment of water may provide an essential piece of
information in the understanding of the formation of comets and protoplanetary
systems, only a few studies up to now have aimed at deriving the HDO/H2O ratio
in low-mass star forming regions. Previous studies of the molecular deuteration
toward the solar-type class 0 protostar, IRAS 16293-2422, have shown that the
D/H ratio of water is significantly lower than other grain-surface-formed
molecules. It is not clear if this property is general or particular to this
source. In order to see if the results toward IRAS 16293-2422 are particular,
we aimed at studying water deuterium fractionation in a second low-mass
solar-type protostar, NGC1333-IRAS2A. Using the 1-D radiative transfer code
RATRAN, we analyzed five HDO transitions observed with the IRAM 30m, JCMT, and
APEX telescopes. We assumed that the abundance profile of HDO in the envelope
is a step function, with two different values in the inner warm (T>100 K) and
outer cold (T<100 K) regions of the protostellar envelope. The inner and outer
abundance of HDO is found to be well constrained at the 3 sigma level. The
obtained HDO inner and outer fractional abundances are x_in=6.6e-8 - 1e-7 and
x_out=9e-11 - 1.8e-9 (3 sigma). These values are close to those in IRAS
16293-2422, which suggests that HDO may be formed by the same mechanisms in
these two solar-type protostars. Taking into account the (rather poorly
constrained) H2O abundance profile deduced from Herschel observations, the
derived HDO/H2O in the inner envelope is larger than 1% and in the outer
envelope it is 0.9%-18%. These values are more than one order of magnitude
higher than what is measured in comets. If the same ratios apply to the
protosolar nebula, this would imply that there is some efficient reprocessing
of the material between the protostellar and cometary phases. The H2O inner
fractional [...]
