arXiv subjects

Taiki Suzuki

Publications and source records attributed to Taiki Suzuki.

8 recordsLinked to original sources

arXiv2024

Chemical Evolution of Complex Organic Molecules in Turbulent Protoplanetary Disks: Effect of stochastic UV irradiation

We investigate the chemical evolution of complex organic molecules (COMs) in turbulent disks using gas-ice chemical reaction network simulations. We trace trajectories of dust particles considering advection, turbulent diffusion, gas drag, and vertical settling, for 10$^6$ yrs in a protoplanetary disk. Then, we solve a gas-ice chemical reaction network along the trajectories and obtain the temporal evolution of molecular abundances. We find that the COM abundances in particles can differ by more than two orders of magnitude even when the UV fluence (i.e., the time integral of UV flux) received by the particles are similar, suggesting that not only the UV fluence but also the time variation of the UV flux does matter for the evolution of COMs in disks. The impact of UV fluence on molecular abundances differs between oxygen-bearing and nitrogen-bearing COMs. While higher UV fluence results in oxygen being locked into CO$_2$, leading to reduced abundances of oxygen-bearing COMs such as CH$_3$OCH$_3$, mild UV exposure can promote their formation by supplying the precursor radicals. On the other hand, nitrogen is not locked up into specific molecules, allowing the formation of nitrogen-bearing COMs, particularly CH$_3$NH$_2$, even for the particle that receives the higher UV fluence. We also find that the final COM abundances are mostly determined by the inherited abundances from the protostellar core when the UV fluence received by dust particles is less than a critical value, while they are set by both the inherited abundances and the chemistry inside the disk at higher UV fluence.

astro-ph.EP↗

arXiv2020

Possibility of Concentration of Non-volatile Species near the Surface of Comet 67P/Churyumov-Gerasimenko

The cometary materials are thought to be the reservoir of primitive materials in the Solar System. The recent detection of glycine and CH$_3$NH$_2$ by the ROSINA mass spectrometer in the coma of 67P/Churyumov-Gerasimenko suggests that amino acids and their precursors may have been formed in an early evolutionary phase of the Solar System. We investigate the evolution of cometary interior considering the evaporation process of water followed by the concentration of non-volatile species. We develop a Simplified Cometary Concentration Model (SCCM) to simulate the evaporation and concentration processes on the cometary surface.We use 67P/Churyumov-Gerasimenko as the benchmark of SCCM. We investigate the depth of the layer where non-volatile species concentrate after the numerous passages of perihelion after the formation of the Solar System. As a result, the SCCM explains the observed production rates of water and CH$_3$NH$_2$ at 100 comet years. SCCM results suggest that the non-volatile species would concentrate at depths between 0 and 100cm of comet surface within 10 comet years. Our results also suggest that the non-volatile species would concentrate several meters beneath the surface before it hit the early Earth. This specific mass of non-volatile species may provide unique chemical condition to the volcanic hot spring pools.

astro-ph.EP↗

arXiv2019

Survey Observation of CH$_3$NH$_2$ and Its Formation Process

We present the observational result of a glycine precursor, methylamine (CH$_3$NH$_2$), together with methanol (CH$_3$OH) and methanimine (CH$_2$NH) towards high-mass star-forming regions, NGC6334I, G10.47+0.03, G31.41+0.3, and W51~e1/e2 using ALMA. The molecular abundances toward these sources were derived using the rotational diagram method and compared with our state-of-the-art chemical model. We found that the observed ratio of "CH$_3$NH$_2$/CH$_3$OH" is in between 0.11 and 2.2. We also found that the observed "CH$_3$NH$_2$/CH$_3$OH" ratio agrees well with our chemical model by considering the formation of CH$_3$NH$_2$ on the grain surface via hydrogenation process to HCN. This result clearly shows the importance of hydrogenation processes to form CH$_3$NH$_2$. NGC63343I MM3, where CH$_3$NH$_2$ was not detected in this study and showed "CH$_3$NH$_2$/CH$_3$OH" ratio of less than 0.02, is clearly distinguished from the other cores.

astro-ph.SR↗

arXiv2018

An Expanded Gas-Grain Model for Interstellar Glycine

The study of the chemical evolution of glycine in the interstellar medium is one of challenging topics in astrochemistry. Here, we present the chemical modeling of glycine in hot cores using the state-of-the-art three-phase chemical model NAUTILUS, which is focused on the latest glycine chemistry. For the formation process of glycine on the grain surface, we obtained consistent results with previous studies that glycine would be formed via the reactions of COOH with CH$_2$NH$_2$. However, we will report three important findings regarding the chemical evolution and the detectability of interstellar glycine. First, with the experimentally obtained binding energy from the temperature programmed thermal desorption (TPD) experiment, a large proportion of glycine was destroyed through the grain surface reactions with NH or CH$_3$O radicals before it fully evaporates. As a result, the formation process in the gas phase is more important than thermal evaporation from grains. If this is the case, NH$_2$OH and CH$_3$COOH rather than CH$_3$NH$_2$ and CH$_2$NH would be the essential precursors to the gas phase glycine. Secondly, since the gas phase glycine will be quickly destroyed by positive ions or radicals, early evolutionary phase of the hot cores would be the preferable target for the future glycine surveys. Thirdly, we suggest the possibility that the suprathermal hydrogen atoms can strongly accelerate the formation of COOH radicals from CO$_2$, resulting in the dramatic increase of formation rate of glycine on grains. The efficiency of this process should be investigated in detail by theoretical and experimental studies in the future.

astro-ph.GA↗

arXiv2018

The Difference in Abundances 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 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.

astro-ph.SR↗

arXiv2017

Detection of New Methylamine (CH$_3$NH$_2$) Sources: Candidates for Future Glycine Surveys

It has been a long-standing problem to detect interstellar glycine (NH$_2$CH$_2$COOH), the simplest amino acid, in studying a possible relation between the Universe and origin of life. In the last about 40 years all surveys of glycine failed, and it would be an alternative strategy to search for precursor(s) to glycine. Such studies of precursors would be crucial prior to conducting sensitive surveys by ALMA. Laboratory studies have suggested that CH$_3$NH$_2$ is a possible precursor to glycine. Further theoretical study also suggested that the CH$_2$NH$_2$ radical that can be formed from CH$_3$NH$_2$ through photodissociation can be a good precursor to glycine. Thus we observed CH$_3$NH$_2$ towards several hot core sources by using the Nobeyama 45m radio telescope, and succeeded in finding a new CH$_3$NH$_2$ source, G10.47+0.03, with its fractional abundance of 2.4+-0.6x10^{-8}; at the time of writing, this source is the most abundant source of CH$_3$NH$_2$ ever known. We also found another new source of CH$_3$NH$_2$, NGC6334F, however, the detection may be tentative since the signal-to-noise ratios are not sufficient to claim secure detection. Detailed analysis of the detected data revealed the detection of the hyperfine components of CH$_3$NH$_2$ for the first time. We found that the observed abundance of CH$_3$NH$_2$ agrees fairly well with the theoretically predicted value by Garrod (2013). Detectability of interstellar glycine is discussed.

astro-ph.GA↗

arXiv2016

Survey Observations of a Possible Glycine Precursor, Methanimine (CH$_2$NH)

We conducted survey observations of a glycine precursor, methanimine or methylenimine (CH$_2$NH), with the NRO 45 m telescope and the SMT telescope towards 12 high-mass and two low-mass star-forming regions in order to increase number of CH$_2$NH sources and to better understand the characteristics of CH2NH sources. As a result of our survey, CH$_2$NH was detected in eight sources, including four new sources. The estimated fractional abundances were ~10$^8$ in Orion KL and G10.47+0.03, while they were ~10$^9$ towards the other sources. Our hydrogen recombination line and past studies suggest that CH$_2$NH-rich sources have less evolved HII regions. The less destruction rates by UV flux from the central star would be contributed to the high CH$_2$NH abundances towards CH$_2$NH-rich sources. Our gas-grain chemical simulations suggest that CH$_2$NH is mostly formed in the gas-phase by neutral-neutral reactions rather than the product of thermal evaporation from the dust surfaces.

astro-ph.GA↗

arXiv2014

Distribution of CCS and HC3N in L1147, an Early Phase Dark Cloud

We used the Nobeyama 45 m radio telescope to reveal spatial distributions of CCS and HC$_3$N in L1147, one of carbon-chain producing regions (CCPRs) candidates, where carbon-chain molecules are dominant rather than NH$_3$. We found that three cores (two CCS cores and one HC$_3$N core) exist along the NE-SW filament traced by the 850 micron dust continuum, which are away from a Very Low Luminosity Object (VeLLO - a source that may turn into sub-stellar mass brown dwarf). The column densities of CCS are 3-7x10$^{12}$ cm$^{-2}$ and those of HC$_3$N are 2-6x10$^{12}$ cm$^{-2}$, respectively, much lower than those previously reported towards other CCPRs. We also found that two CCS peaks are displaced from that of HC$_3$N. In order to interpret such interleaved distributions, we conducted chemical reaction network simulations, and found that slightly different gas densities could lead to large variation of CCS-to-HC$_3$N ratio in the early phase of dark cloud evolution. Such a chemical "variation" may be seen in other CCPRs. Finally we were able to confirm that the L1147 filament can be regarded as a CCPR.

astro-ph.SR↗