Survey Observations of a Possible Glycine Precursor, Methanimine (CH$_2$NH)

TL;DR

This survey identified multiple sources of methanimine, a glycine precursor, revealing its abundance patterns and suggesting gas-phase formation mechanisms over dust surface evaporation in star-forming regions.

Contribution

The study expands the catalog of methanimine sources and provides insights into its formation pathways and relation to star formation stages.

Findings

Methanimine detected in 8 out of 14 surveyed regions.

Higher methanimine abundance in less evolved HII regions.

Gas-phase neutral-neutral reactions are primary formation pathways.

Abstract

We conducted survey observations of a glycine precursor, methanimine or methylenimine (CH$_2$NH), with the NRO 45 m telescope and the SMT telescope towards 12 high-mass and two low-mass star-forming regions in order to increase number of CH$_2$NH sources and to better understand the characteristics of CH2NH sources. As a result of our survey, CH$_2$NH 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$^9$ towards the other sources. Our hydrogen recombination line and past studies suggest that CH$_2$NH-rich sources have less evolved HII regions. The less destruction rates by UV flux from the central star would be contributed to the high CH$_2$NH abundances towards CH$_2$NH-rich sources. Our gas-grain chemical simulations suggest that CH$_2$NH is mostly formed in the gas-phase by neutral-neutral reactions rather than the product of thermal evaporation from the dust surfaces.