2023.6.12 highlights

Reveal chemical links involving NH2CHO that contains a peptide bond around high-mass protostars

An international team led by Dr. Kotomi Taniguchi (Division of Science, NAOJ) and Dr. Patricio Sanhueza (ALMA Project, NAOJ) has published a paper using a more advanced statistical method to investigate chemical links among complex organic molecules (COMs) in the Universe.

Formamide (NH2CHO) is the simplest amide containing a peptide bond, which is important to form proteins from amino acids. Formamide has been regarded as a prebiotic molecule for a long time. The formation and destruction processes of NH2CHO are important to understand how prebiotic molecules can form from simple molecules in the Universe. However, its formation process in the Universe is controversial. Several possible formation pathways of NH2CHO have been considered; e.g., successive hydrogenation reactions of HNCO on dust surfaces, and the gas-phase reaction between H2CO and NH2. One of the possible solutions to investigate the formation processes of molecules is to investigate correlations among molecules, which are likely pairs of parent species and daughter species. The methods that have been used for the investigation of correlations among COMs formed on dust surfaces suffer from a third variable that the interpretation of the observed correlations difficult. Thus, statistical studies need to be revisited.

Dr. Sanhueza leads the Digging into the Interior of Hot Cores with ALMA (DIHCA) survey project, which covers 30 high-mass star-forming regions with ALMA Band 6. In Taniguchi et al. (2023) published in ApJ, we have analyzed the NH2CHO, HNCO, H2CO, and CH3CN lines obtained by the DIHCA project. In general, NH2CHO and HNCO show more spatially compact emission compared to that of CH3CN and H2CO (Figure 1). We have derived molecular abundances at each hot molecular core (HMC) and investigated correlations among the previously mentioned molecules. We have found strong correlations between NH2CHO and HNCO and between NH2CHO and H2CO (Figure 2). In order to confirm their chemical links, we have applied a partial correlation test so that we can exclude the temperature effect. These pairs of molecules show high correlation coefficients (ρ=0.89 and 0.84 for pairs of NH2CHO-HNCO and NH2CHO-H2CO, respectively). Thus, we have clearly shown that these three species are tightly chemically linked in the hot dense gas around high-mass protostars. We propose that partial correlation tests are useful to investigate correlations among COMs formed on dust surfaces.

We have run chemical simulations to find out how they are linked. The HNCO molecules convert to NH2CHO molecules by hydrogen-addition reactions, and NH2CHO can go back to HNCO by hydrogen abstraction reactions. This mechanism is called the dual-cyclic hydrogen addition and abstraction reactions. In addition, NH2CHO can be formed by the gas-phase reaction between H2CO and NH2 in the hot dense gas. Figure 3 shows the schematic view of relationships among these three species around high-mass protostars.

Figure1. Continuum images (λ=1.33 mm, gray scales) overlaid with contours indicating moment 0 maps of molecular lines (left panel: white; CH3CN and cyan; H2CO, right panel: magenta; NH2CHO and yellow; HNCO) toward G10.62-0.38. Red crosses indicate the positions of hot molecular cores (HMCs) identified based on moment 0 maps of the CH3CN line.
Figure2. Abundance correlations between NH2CHO and HNCO (left panel) and NH2CHO and H2CO (right panel). Red lines show the best power-law fits for each pair.
Figure3. Schematic view of chemical links among NH2CHO, HNCO, and H2CO in hot molecular cores.

Related Links:

arXiv: http://arxiv.org/abs/2304.00267

Paper Information;  Kotomi Taniguchi et al 2023 ApJ 950 57

Link for the paper; https://iopscience.iop.org/article/10.3847/1538-4357/acca1d