First detection of NHD and ND$_2$ in the interstellar medium
  Deuterium fractionation processes in the interstellar medium (ISM) have been
shown to be highly efficient in the family of nitrogen hydrides. To date,
observations were limited to ammonia (NH$_2$D, NHD$_2$, ND$_3$) and imidogen
radical (ND) isotopologues. We want to explore the high frequency windows
offered by the \emph{Herschel Space Observatory} to search for deuterated forms
of amidogen radical NH$_2$ and to compare the observations against the
predictions of our comprehensive gas-grain chemical model. Making use of the
new molecular spectroscopy data recently obtained at high frequencies for NHD
and ND$_2$, both isotopologues have been searched for in the spectral survey
towards the class 0 IRAS 16293-2422, a source in which NH$_3$, NH and their
deuterated variants have been previously detected. We used the observations
carried out with HIFI (Heterodyne Instrument for the Far Infrared) in the
framework of the key program "Chemical Herschel surveys of star forming
regions" (CHESS). We report the first detection of interstellar NHD and ND$_2$.
Both species are observed in absorption against the continuum of the protostar.
From the analysis of their hyperfine structure, accurate excitation temperature
and column density values have been determined. The latter were combined with
the column density of the parent species NH$_2$ to derive the deuterium
fractionation in amidogen. The amidogen D/H ratio measured in the low-mass
protostar IRAS 16293-2422 is comparable to the one derived for the related
species imidogen and much higher than that observed for ammonia. Additional
observations of these species will give more insights into the mechanism of
ammonia formation and deuteration in the ISM. We finally indicate the current
possibilities to further explore these species at submillimeter wavelengths.
