2009A&A...498L...9G

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 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 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 obtained deep integrations of the ortho ¹⁵NH₂D (1_1,1-1_0,1) line at 86.4GHz, simultaneously with the corresponding ortho NH₂D line at 85.9GHz. The ortho ¹⁵NH₂D (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 ¹⁴N/¹⁵N abundance ratio in ¹⁵NH₂D ranges between 350 and 850, similar to the protosolar value of ∼424, and likely higher than the terrestrial ratio of ∼270.