The Difference in Abundance between N-bearing and O-bearing Species in High-mass Star-forming Regions

The different spatial distributions of N-bearing and O-bearing species, as are well known toward Orion KL, is a longstanding mystery. 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 toward eight star-forming regions. Through our spectral survey ranging from 80 to 108 GHz, we detected CH3OH, HCOOCH3, CH3OCH3, (CH3)2CO, CH3CHO, CH3CH2CN, CH2CHCN, and NH2CHO. 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 NGC 6334F were not so rich, being less than 0.01 compared to CH3OH. Then the molecular abundances toward these sources were evaluated by chemical modeling with the NAUTILUS three-phase gas-grain chemical code. Based on the simulations of time evolution for the abundance of COMs, we suggest that the observed correlations of fractional abundances between COMs can be explained by a combination of different temperature structures inside the hot cores and the different evolutionary phases. 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.