Detection of 15NH2D in dense cores: A new tool for measuring the 14N/15N
  ratio in the cold ISM
  Ammonia is one of the best tracers of cold dense cores. It is also a minor
constituent of interstellar ices and, as such, one of the important nitrogen
reservoirs in the protosolar nebula, together with the gas phase nitrogen, in
the form of N2 and N. An important diagnostic of the various nitrogen sources
and reservoirs of nitrogen in the Solar System is the 14N/15N isotopic ratio.
While good data exist for the Solar System, corresponding measurements in the
interstellar medium are scarce and of low quality. Following the successful
detection of the singly, doubly, and triply deuterated isotopologues of
ammonia, we have searched for 15NH2D in dense cores, as a new tool for
investigating the 14N/15N ratio in dense molecular gas. With the IRAM-30m
telescope, we have obtained deep integrations of the ortho 15NH2D
(1(1,1)-1(0,1)) line at 86.4 GHz, simultaneously with the corresponding ortho
NH2D line at 85.9 GHz. o-15NH2D is detected in Barnard-1b, NGC1333-DCO+, and
L1689N, while we obtained upper limits towards LDN1544 and NGC1333-IRAS4A, and
a tentative detection towards L134N(S). The 14N/15N abundance ratio in NH2D
ranges between 350 and 850, similar to the protosolar value of ~ 424, and
likely higher than the terrestrial ratio of 270.
