2016ApJ...825...79S

We conducted survey observations of a glycine precursor, methanimine, or methylenimine (CH₂NH), with the Nobeyama Radio Observatory 45 m telescope and the Sub-Millimeter Radio telescope toward 12 high-mass and two low-mass star-forming regions in order to increase the number of known CH₂NH sources and to better understand the characteristics of CH₂NH sources. As a result of our survey, CH₂NH was detected in eight sources, including four new sources. The estimated fractional abundances were ∼10^–8 in Orion KL and G10.47+0.03, while they were ∼10^–9 toward the other sources. Our hydrogen recombination line and past studies suggest that CH₂NH-rich sources have less (this mean not so evolved) evolved H ii regions. The lower destruction rates from UV flux from the central star would contribute to the high CH₂NH abundances toward CH₂NH-rich sources. Our gas-grain chemical simulations suggest that CH₂NH is mostly formed in the gas phase by neutral-neutral reactions, rather than being the product of thermal evaporation from dust surfaces.