Letter to the Editor

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

¹ LERMA, UMR 8112, CNRS, Observatoire de Paris and École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France e-mail: maryvonne.gerin@ens.fr
² Laboratorio de Astrofísica Molecular, CAB-CSIC/INTA, Ctra de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain e-mail: nuria@damir.iem.csic.es
³ LESIA, UMR 8109, CNRS and Observatoire de Paris, 5 place J. Janssen, 92195 Meudon Cedex, France e-mail: [nicolas.biver;dominique.bockelee]@obspm.fr
⁴ LUTh, Observatoire de Paris and UMR 8102 CNRS, 5 place J. Janssen, 92195 Meudon Cedex, France e-mail: evelyne.roueff@obspm.fr
⁵ LISA, UMR 7583 CNRS and Université Paris 12, 61 avenue du Général de Gaulle, 94010 Créteil Cedex, France e-mail: coudert@lisa.univ-paris12.fr
⁶ California Institute of Technology, MC 320-47, Pasadena, CA 91125, USA e-mail: dcl@caltech.edu

Received: 30 January 2009
Accepted: 12 March 2009

Abstract

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 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.

Aims. 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.

Methods. With the IRAM-30 m telescope, we obtained deep integrations of the ortho ¹⁵NH₂D (–) line at 86.4 GHz, simultaneously with the corresponding ortho NH₂D line at 85.9 GHz.

Results. The ortho 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 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.