The Difference in Abundances between N-bearing and O-bearing Species in High-Mass Star-Forming Regions

TL;DR

This study investigates the spatial distribution and chemical abundances of N-bearing and O-bearing complex organic molecules in high-mass star-forming regions, revealing correlations influenced by temperature structures and evolutionary phases.

Contribution

It provides the first extensive spectral survey of COMs across multiple star-forming regions and combines observational data with chemical modeling to explore molecular distribution patterns.

Findings

N-bearing molecules show stronger correlations with other N-bearing species.

High fractional abundances of N-bearing species vary between regions like G10.47+0.03 and NGC6334F.

Chemical modeling suggests temperature and evolutionary phase influence molecular abundance correlations.

Abstract

The different spatial distributions of N-bearing and O-bearing species, as is well known towards Orion~KL, is one of the long-lasting mysteries. We conducted a survey observation and chemical modeling study to investigate if the different distributions of O- and N-bearing species are widely recognized in general star-forming regions. First, we report our observational results of complex organic molecules (COMs) with the 45~m radio telescope at the Nobeyama Radio Observatory towards eight star-forming regions. Through our spectral survey ranging from 80 to 108~GHz, we detected CH$_3$OH, HCOOCH$_3$, CH$_3$OCH$_3$, (CH$_3$)$_2$CO, CH$_3$CHO, CH$_3$CH$_2$CN, CH$_2$CHCN, and NH$_2$CHO. Their molecular abundances were derived via the rotation diagram and the least squares methods. We found that N-bearing molecules, tend to show stronger correlations with other N-bearing molecules rather than O-bearing molecules. While G10.47+0.03 showed high fractional abundances of N-bearing species, those in NGC6334F were not so rich, being less than 0.01 compared to CH$_3$OH. Then, the molecular abundances towards these sources were evaluated by chemical modeling with NAUTILUS three-phase gas-grain chemical code. Through the simulations of time evolutions for the abundances of COMs, we suggest that observed correlations of fractional abundances between COMs can be explained by the combination of the different temperature structures inside the hot cores and the different evolutionary phase. Since our modeling could not fully explain the observed excitation temperatures, it is important to investigate the efficiency of grain surface reactions and their activation barriers, and the binding energy of COMs to further promote our understanding.