In:
Astronomy & Astrophysics, EDP Sciences, Vol. 624 ( 2019-4), p. A82-
Abstract:
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 (CH 3 NH 2 ) 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 CH 3 NH 2 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 1.3 kpc. 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 CH 3 NH 2 , CH 2 NH, NH 2 CHO, and the 13 C- and 15 N-methyl cyanide (CH 3 CN) isotopologues, detected towards NGC 6334I. Column densities are derived for each species assuming local thermodynamic equilibrium and excitation temperatures in the range 220–340 K for CH 3 NH 2 , 70–110 K for the CH 3 CN isotopologues and 120–215 K for NH 2 CHO and CH 2 NH. Results. We report the first detections of CH 3 NH 2 towards NGC 6334I with column density ratios with respect to CH 3 OH of 5.9 × 10 −3 , 1.5 × 10 −3 and 5.4 × 10 −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 NH 2 CHO, 13 CH 3 CN, and CH 3 C 15 N with respect to CH 3 OH are (1.5 – 1.9) × 10 −4 , (1.0 – 4.6) × 10 −3 and (1.7 – 3.0) × 10 −3 respectively. Lower limits of 5.2, 1.2, and 3.0 are reported for the CH 3 NH 2 to CH 2 NH 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 CH 3 NH 2 in the hot cores of NGC 6334I hint that CH 3 NH 2 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 CH 3 NH 2 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 CH 3 NH 2 chemistry depends on the temperature of the grains in high- and low-mass star-forming regions respectively.
Type of Medium:
Online Resource
ISSN:
0004-6361
,
1432-0746
DOI:
10.1051/0004-6361/201833676
Language:
English
Publisher:
EDP Sciences
Publication Date:
2019
detail.hit.zdb_id:
1458466-9
SSG:
16,12
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