Nitrogen hydrides in the cold envelope of IRAS16293-2422
  Nitrogen is the fifth most abundant element in the Universe, yet the
gas-phase chemistry of N-bearing species remains poorly understood. Nitrogen
hydrides are key molecules of nitrogen chemistry. Their abundance ratios place
strong constraints on the production pathways and reaction rates of
nitrogen-bearing molecules. We observed the class 0 protostar IRAS16293-2422
with the heterodyne instrument HIFI, covering most of the frequency range from
0.48 to 1.78~THz at high spectral resolution. The hyperfine structure of the
amidogen radical o-NH2 is resolved and seen in absorption against the continuum
of the protostar. Several transitions of ammonia from 1.2 to 1.8~THz are also
seen in absorption. These lines trace the low-density envelope of the
protostar. Column densities and abundances are estimated for each hydride. We
find that NH:NH2:NH3=5:1:300. {Dark clouds chemical models predict steady-state
abundances of NH2 and NH3 in reasonable agreement with the present
observations, whilst that of NH is underpredicted by more than one order of
magnitude, even using updated kinetic rates. Additional modelling of the
nitrogen gas-phase chemistry in dark-cloud conditions is necessary before
having recourse to heterogen processes.
