Probable detection of interstellar methylamine-D /CH3NHD/

Abstract

The letter reports the detection in three sources, including the galactic center, of strong lines tentatively attributed to the 2(02)(+) - 1(10)(-) A-state transition of CH3NHD on the basis of frequency coincidence. Laboratory measurements of the spectra of CH3NH2 and CH3NHD are discussed, and it is noted that the detected molecular lines were observed in absorption toward Sgr A and Sgr B2 but in emission toward Ori A. Velocity widths are derived for the profiles of each source, unsuccessful searches and a marginal detection toward W51 are briefly reviewed, and the excitation of CH3NHD in each source is analyzed. The CH3NHD/CH3NH2 concentration ratio is determined to be between 1 and 30 in the direction of Sgr B2. Two different mean ranges of the D/H concentration ratio in the same direction, including one that lies between the terrestrial and primordial ratios, are obtained by assuming that the very large enrichment of CH3NHD results from a gas-phase chemical reaction or a reaction taking place on interstellar grains.