10:15 AM - 10:30 AM
[PCG21-06] Experiments on Photochemical Synthesis of Hexamethylenetetramine (HMT) and its derivatives in Interstellar Ice with different compositions
Keywords:Interstellar chemistry, photochemistry, low temperature science, molecular cloud, organics
Various organic molecules are detected by the radio astronomy in star and planetary system forming regions. Complex organic molecules (COMs) containing six or more atoms are crucial for understanding of the chemical evolution of organics (Tielens, 2013). The D enrichments in some extraterrestrial organic matter in chondrites and returned asteroid samples suggest that COMs were delivered from the cold environment (Yabuta et al. 2023).
The photochemical synthesis of COMs in the ice mantle on the interstellar dust grains has been proposed as the main formation pathway of COMs. The ice mantle contain mainly water along with CO2, CO, CH3OH, NH3, and CH4(Boogert et al. 2015). Photodissociation of ice-forming molecules form radials, which begin to diffuse and react to form diverse COMs with increasing temperature by the formation of protostar and so on. By elucidating this formation process and comparing to with extraterrestrial materials, it would be possible to constrain the physicochemical environment during the formation of the solar system.
To understand the interstellar photochemistry, various experiments on photochemistry of interstellar ice analogs have been conducted. Hexamethylenetetramine (HMT) and its derivatives are important products in the photochemical experiments (Muñoz Caro et al. 2002), and they have been found in Murchison (CM2) meteorite (Oba et al. 2020). They are precursors for amino acids and N-bearing organic molecules (Hulett et al. 1971). HMT and its derivatives have been detected from experiments with various compositions of ice, but changes in the production ratio of them depending on the initial gas are poorly understood. Furthermore, HMT derivatives can be potential targets for radio astronomy because they have dipole moments.
In this study, we studied the photochemistry to form HMT and its derivatives by using different N- and C-reservoirs (N2 and NH3 for N; CH3OH and CH4 for C).
We used the instrument called PICACHU (Photochemistry in Interstellar Cloud for Astro-Chronicle in Hokkaido University/Habitable Universe) to synthesize the interstellar ice analogues (Piani et al. 2017; Tachibana et al. 2017; Orthus-Daunay et al. 2019; Sugahara et al. 2019). The ice was formed by deposition of mixed gases with the solar C/N/O ratios onto a gold substrate at ~25 K with simultaneous UV irradiation from deuterium lamps. The chamber pressure was ~10-5 Pa with a minor leak of air, resulting in the deposition of N2 and O2 as well, which was used as the additional N2 source in this study. After the gas deposition, the substrate was warmed to room temperature.
The substrate was then wiped with glass wool soaked with ultrapure water and methanol. The extracts were analyzed using a high-resolution Orbitrap mass spectrometer (Q Exactive Orbitrap).
HMT and its derivatives were not detected in the sample without NH3 but with N2, suggesting that N2 has little of no contribution for the synthesis. HMT and its derivatives were detected in the sample containing NH3 and CH3OH. The main derivatives were HMT-CH3 and HMT-OH. HMT-CH3 was less abundant than HMT (HMT-CH3/HMT ~0.1). HMT and its derivatives were detected in the sample containing NH3 and CH4 as well, but HMT-CH3 was more abundant than HMT (HMT-CH3/HMT ~100).
HMT-CH3/HMT is ~0.02 in Murchison meteorite (Oba et al. 2020), suggesting that CH4, was poor in the ice in the Sun’s parent molecular cloud if the HMT-CH3/HMT was not affected by aqueous alteration on the parent body of Murchison.
When the molecular cloud has sufficient time to convert atomic C to CO through gas-phase reactions, CH3OH form via hydrogeneration of CO adsorbed onto dust surfaces. On the other hand, when the conversion of C to CO does not occur sufficiently, and a large fraction of C atoms adsorbs on the dust grains to form CH4 by hydrogeneration (Sakai et al. 2009). The present result indicates that the Sun’s parent molecular cloud had sufficient time to convert C to CO.
The photochemical synthesis of COMs in the ice mantle on the interstellar dust grains has been proposed as the main formation pathway of COMs. The ice mantle contain mainly water along with CO2, CO, CH3OH, NH3, and CH4(Boogert et al. 2015). Photodissociation of ice-forming molecules form radials, which begin to diffuse and react to form diverse COMs with increasing temperature by the formation of protostar and so on. By elucidating this formation process and comparing to with extraterrestrial materials, it would be possible to constrain the physicochemical environment during the formation of the solar system.
To understand the interstellar photochemistry, various experiments on photochemistry of interstellar ice analogs have been conducted. Hexamethylenetetramine (HMT) and its derivatives are important products in the photochemical experiments (Muñoz Caro et al. 2002), and they have been found in Murchison (CM2) meteorite (Oba et al. 2020). They are precursors for amino acids and N-bearing organic molecules (Hulett et al. 1971). HMT and its derivatives have been detected from experiments with various compositions of ice, but changes in the production ratio of them depending on the initial gas are poorly understood. Furthermore, HMT derivatives can be potential targets for radio astronomy because they have dipole moments.
In this study, we studied the photochemistry to form HMT and its derivatives by using different N- and C-reservoirs (N2 and NH3 for N; CH3OH and CH4 for C).
We used the instrument called PICACHU (Photochemistry in Interstellar Cloud for Astro-Chronicle in Hokkaido University/Habitable Universe) to synthesize the interstellar ice analogues (Piani et al. 2017; Tachibana et al. 2017; Orthus-Daunay et al. 2019; Sugahara et al. 2019). The ice was formed by deposition of mixed gases with the solar C/N/O ratios onto a gold substrate at ~25 K with simultaneous UV irradiation from deuterium lamps. The chamber pressure was ~10-5 Pa with a minor leak of air, resulting in the deposition of N2 and O2 as well, which was used as the additional N2 source in this study. After the gas deposition, the substrate was warmed to room temperature.
The substrate was then wiped with glass wool soaked with ultrapure water and methanol. The extracts were analyzed using a high-resolution Orbitrap mass spectrometer (Q Exactive Orbitrap).
HMT and its derivatives were not detected in the sample without NH3 but with N2, suggesting that N2 has little of no contribution for the synthesis. HMT and its derivatives were detected in the sample containing NH3 and CH3OH. The main derivatives were HMT-CH3 and HMT-OH. HMT-CH3 was less abundant than HMT (HMT-CH3/HMT ~0.1). HMT and its derivatives were detected in the sample containing NH3 and CH4 as well, but HMT-CH3 was more abundant than HMT (HMT-CH3/HMT ~100).
HMT-CH3/HMT is ~0.02 in Murchison meteorite (Oba et al. 2020), suggesting that CH4, was poor in the ice in the Sun’s parent molecular cloud if the HMT-CH3/HMT was not affected by aqueous alteration on the parent body of Murchison.
When the molecular cloud has sufficient time to convert atomic C to CO through gas-phase reactions, CH3OH form via hydrogeneration of CO adsorbed onto dust surfaces. On the other hand, when the conversion of C to CO does not occur sufficiently, and a large fraction of C atoms adsorbs on the dust grains to form CH4 by hydrogeneration (Sakai et al. 2009). The present result indicates that the Sun’s parent molecular cloud had sufficient time to convert C to CO.