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Context. In the search for the building blocks of life, nitrogen-bearing molecules are of particular interest since nitrogen-containing bonds are essential for the linking of amino acids and ultimately the formation of larger biological structures. The elusive molecule methylamine (CH₃NH₂) is thought to be a key pre-biotic species but has so far only been securely detected in the giant molecular cloud Sagittarius B2.
Aims. We identify CH₃NH₂ and other simple nitrogen-bearing species involved in the synthesis of biologically relevant molecules towards three hot cores associated with the high-mass star-forming region NGC 6334I, located at a distance of 1.3kpc. Column density ratios are derived in order to investigate the relevance of the individual species as precursors of biotic molecules.
Methods. High sensitivity, high angular and spectral resolution observations obtained with the Atacama Large Millimeter/ submillimeter Array were used to study transitions of CH₃NH₂, CH₂NH, NH₂CHO, and the ¹³C- and ¹⁵N-methyl cyanide (CH₃CN) isotopologues, detected towards NGC 6334I. Column densities are derived for each species assuming local thermodynamic equilibrium and excitation temperatures in the range 220-340K for CH₃NH₂, 70-110K for the CH₃CN isotopologues and 120-215K for NH₂CHO and CH₂NH.
Results. We report the first detections of CH₃NH₂ towards NGC 6334I with column density ratios with respect to CH₃OH of 5.9x10^–3, 1.5x10^–3 and 5.4x10^–4 for the three hot cores MM1, MM2, and MM3, respectively. These values are slightly lower than the values derived for Sagittarius B2 but higher by more than an order of magnitude as compared with the values derived for the low-mass protostar IRAS 16293-2422B. The column density ratios of NH₂CHO, ¹³CH₃CN, and CH₃C¹⁵N with respect to CH₃OH are (1.5-1.9)x10^–4, (1.0-4.6)x10^–3 and (1.7-3.0)x10^–3 respectively. Lower limits of 5.2, 1.2, and 3.0 are reported for the CH₃NH₂ to CH₂NH column density ratio for MM1, MM2, and MM3 respectively. These limits are largely consistent with the values derived for Sagittarius B2 and higher than those for IRAS 16293-2422B.
Conclusions. The detections of CH₃NH₂ in the hot cores of NGC 6334I hint that CH₃NH₂ is generally common in the interstellar medium, albeit that high-sensitivity observations are essential forthe detection of the species. The good agreement between model predictions of CH₃NH₂ ratios and the observations towards NGC 6334I indicate a main formation pathway via radical recombination on grain surfaces. This process may be stimulated further by high grain temperatures allowing a lager degree of radical mobility. Further observations with ALMA will help evaluate the degree to which CH₃NH₂ chemistry depends on the temperature of the grains in high- and low-mass star-forming regions respectively.