[MIS07-P12] Formation Process of A Glycine's Precursor Candidate, CH3NH2
キーワード：電波天文学、化学モデル、生命関連分子
As [1] argued, the comets and meteorites may have provided a large amount of prebiotic molecules to the early Earth. An attempt to understand the origins of life must necessarily begin with detailed studies of the formation and evolution of complex organic molecules (COMs), products of complex chemistry that most likely starts in molecular clouds and continues within the protoplanetary disk.
Since the amino acids are the essential building blocks of life, the search for amino acids and their complex organic precursors at different stages of star and planet formation is one of the exciting topics in modern astronomy. Recently, volatile glycine (NH2CH2COOH) was detected in the coma of comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer [2], also giving clues about glycine's interstellar origin. However, the survey observations of glycine toward ISM have been not successful. Therefore, revealing the formation pathways to glycine is important for astrochemistry and astrobiology not only to know the promising glycine-rich sources for future surveys but also to understand the general chemical evolution of N-bearing molecules in the ISM.
In this context, we performed the chemical modeling study for the high-mass star, with the possible formation processes of glycine with theoretically and/or experimentally suggested formation paths [3]. We suggested that glycine was efficiently formed through the following reactions: First, the hydrogenation process to HCN leads to the formation of CH2NH and CH3NH2 on interstellar grain surface (HCN + 2H => CH2NH, and then CH2NH + 2H => CH3NH2). Then, the destruction of CH3NH2 by radicals will form CH2NH2. Finally, CH2NH2 and COOH radical react to form glycine. For this test, a key molecule would be CH3OH, whose formation process is well known. Both experiments and theoretical works suggested that CH3OH is built via grain surface hydrogenation processes to CO (CO + 2H => H2CO, and then H2CO + 2H => CH3OH). If CH3NH2 is also produced through hydrogenation process, “CH3NH2/CH3OH” ratio should be well reproduced by the modeling. However, it was difficult to test this formation process due to the very limited number of previous CH3NH2 observations.
With this motivation, we performed the observation of CH3NH2 and CH3OH with ALMA toward high mass star-forming regions, G10.47+0.03, NGC6334I, W51 e1/e2, and G31.41+0.3. As a result, both of CH3OH and CH3NH2 were detected toward all sources. The molecular abundances were derived with rotation diagram method. The high excitation temperatures of 100 and 200 K and compact distributions suggest that CH3NH2 emissions originate from the hot and dense regions. The observed molecular abundance ratio, “CH3NH2/CH3OH” ranges from 0.2 to 2.1. The low “CH3NH2/CH3OH” ratio of 0.2 is close to our modeling, supporting that CH3NH2 is the product of the hydrogenation process in those sources. However, the higher “CH3NH2/CH3OH” ratio of nearly two, observed toward NGC6334I MM1, could not be reproduced with our modeling under the possible parameter range. Thus, our chemical modeling would not correctly reproduce the physics or chemistry of such CH3NH2-rich sources. Further studies of such sources will enable us to promote our knowledge of glycine chemistry and provide us with promising sources for future glycine surveys.
Citations:
[1] Ehrenfreund (2002), Rep. Prog. Phys. 65 1427
[2] Altwegg et al. (2016), Sci, 2, e1600285
[3] Suzuki et al, (2018), ApJ, 825, 51
Since the amino acids are the essential building blocks of life, the search for amino acids and their complex organic precursors at different stages of star and planet formation is one of the exciting topics in modern astronomy. Recently, volatile glycine (NH2CH2COOH) was detected in the coma of comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer [2], also giving clues about glycine's interstellar origin. However, the survey observations of glycine toward ISM have been not successful. Therefore, revealing the formation pathways to glycine is important for astrochemistry and astrobiology not only to know the promising glycine-rich sources for future surveys but also to understand the general chemical evolution of N-bearing molecules in the ISM.
In this context, we performed the chemical modeling study for the high-mass star, with the possible formation processes of glycine with theoretically and/or experimentally suggested formation paths [3]. We suggested that glycine was efficiently formed through the following reactions: First, the hydrogenation process to HCN leads to the formation of CH2NH and CH3NH2 on interstellar grain surface (HCN + 2H => CH2NH, and then CH2NH + 2H => CH3NH2). Then, the destruction of CH3NH2 by radicals will form CH2NH2. Finally, CH2NH2 and COOH radical react to form glycine. For this test, a key molecule would be CH3OH, whose formation process is well known. Both experiments and theoretical works suggested that CH3OH is built via grain surface hydrogenation processes to CO (CO + 2H => H2CO, and then H2CO + 2H => CH3OH). If CH3NH2 is also produced through hydrogenation process, “CH3NH2/CH3OH” ratio should be well reproduced by the modeling. However, it was difficult to test this formation process due to the very limited number of previous CH3NH2 observations.
With this motivation, we performed the observation of CH3NH2 and CH3OH with ALMA toward high mass star-forming regions, G10.47+0.03, NGC6334I, W51 e1/e2, and G31.41+0.3. As a result, both of CH3OH and CH3NH2 were detected toward all sources. The molecular abundances were derived with rotation diagram method. The high excitation temperatures of 100 and 200 K and compact distributions suggest that CH3NH2 emissions originate from the hot and dense regions. The observed molecular abundance ratio, “CH3NH2/CH3OH” ranges from 0.2 to 2.1. The low “CH3NH2/CH3OH” ratio of 0.2 is close to our modeling, supporting that CH3NH2 is the product of the hydrogenation process in those sources. However, the higher “CH3NH2/CH3OH” ratio of nearly two, observed toward NGC6334I MM1, could not be reproduced with our modeling under the possible parameter range. Thus, our chemical modeling would not correctly reproduce the physics or chemistry of such CH3NH2-rich sources. Further studies of such sources will enable us to promote our knowledge of glycine chemistry and provide us with promising sources for future glycine surveys.
Citations:
[1] Ehrenfreund (2002), Rep. Prog. Phys. 65 1427
[2] Altwegg et al. (2016), Sci, 2, e1600285
[3] Suzuki et al, (2018), ApJ, 825, 51