Stratified NH and ND emission in the prestellar core 16293E in L1689N
  High degrees of deuterium fractionation are commonly found in cold prestellar
cores and in the envelopes around young protostars. As it brings strong
constraints to chemical models, deuterium chemistry is often used to infer core
history or molecule formation pathways. Whereas a large number of observations
is available regarding interstellar deuterated stable molecules, relatively
little is known about the deuteration of hydride radicals, as their fundamental
rotational transitions are at high frequencies where the atmosphere is mostly
opaque. Nitrogen hydride radicals are important species in nitrogen chemistry,
as they are thought to be related to ammonia formation. Observations have shown
that ammonia is strongly deuterated, with [NH2D]/[NH3] ~ 10%. Models predict
similarly high [ND]/[NH] ratios, but so far only one observational
determination of this ratio is available, towards the envelope of the protostar
IRAS16293-2422. In order to test model predictions, we aim here at determining
[ND]/[NH] in a dense, starless core. We observed NH and ND in 16293E with the
HIFI spectrometer on board the Herschel Space Observatory as part of the CHESS
guaranteed time key programme, and derived the abundances of these two species
using a non-LTE non-local radiative transfer model. Both NH and ND are detected
in the source, with ND in emission and NH in absorption against the continuum
arising from the cold dust emission. Our model shows however that the ND
emission and the NH absorption originate from different layers in the cloud, as
further evidenced by their different velocities. In the central region of the
core, we can set a lower limit to the [ND]/[NH] ratio of ~2%. This estimate is
consistent with recent pure gas-phase models of nitrogen chemistry
