Detection of 15NH2D in dense cores: a new tool for measuring the 14N/15N ratio in the cold ISM

Context. Ammonia is one of the most reliable 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 N-2 and N. An important diagnostic of the various nitrogen sources and reservoirs of nitrogen in the Solar System is the N-14/N-15 isotopic ratio. While good data exist for the Solar System, corresponding measurements in the interstellar medium are scarce and of low quality. Aims. Following the successful detection of the singly, doubly, and triply deuterated isotopologues of ammonia, we searched for (NH2D)-N-15 in dense cores, as a new tool for investigating the N-14/N-15 ratio in dense molecular gas. Methods. With the IRAM-30m telescope, we obtained deep integrations of the ortho (NH2D)-N-15 (1(1,1)-1(0,1)) line at 86.4 GHz, simultaneously with the corresponding ortho NH2D line at 85.9 GHz. Results. The ortho (NH2D)-N-15 (1(1,0)-1(0,1)) 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 para line at 109 GHz remains undetected at the rms noise level achieved. The N-14/N-15 abundance ratio in (NH2D)-N-15 ranges between 350 and 850, similar to the protosolar value of similar to 424, and likely higher than the terrestrial ratio of similar to 270.