An Expanded Gas-grain Model for Interstellar Glycine

To investigate the chemistry of glycine in hot cores, we have used the state-of-the-art chemical code NAUTILUS described in Ruaud et al. (2016). NAUTILUS allows us to compute the time evolution of chemical abundances for a given set of physical and chemical parameters. It simulates chemistry in three phases, i.e., gas phase, grain surface, and grain mantle. It also considers various possible exchanges among the different phases via adsorption of gas-phase species onto grain surfaces, the thermal and nonthermal desorption of species from grain surface into the gas phase, and the surface–mantle and mantle–surface exchange of species. A gas-to-dust ratio of 100 by mass, a grain density of 3 g cm⁻³, and a grain radius of 10⁻⁵ cm were employed. Then, the total surface area was calculated assuming compact spherical grains. We modeled the standard interstellar UV radiation field and its destructions of molecules as was presented by Ruaud et al. (2016). Competition between reaction, diffusion, and evaporation is also taken into account in the model by following Chang et al. (2007) and Garrod et al. (2007). NAUTILUS computes diffusion energies of each species as a fraction of their binding energies and assumes a ratio of 0.4 for the surface and 0.8 for the ice mantle (see Ruaud et al. 2016, for more discussions). The thickness of the assumed barrier width is 1 Å that a surface species needs to cross while undergoing quantum tunneling to diffuse from one surface site to another. We have also updated the binding energy of each species in our model from Wakelam et al. (2017). The binding energy of a species is typically obtained by temperature-programmed desorption (TPD) experiments where either the temperature of the substrate is kept constant while the species of interest is deposited or the temperature is linearly increased until the species are desorbed from the substrate. Deposited species on the substrate can be in either the monolayer or multilayer regime. In the multilayer regime, desorption occurs from the species itself, and the impact of the underlying substrate becomes negligible as pointed out by Green et al. (2009). For glycine, the binding energy is estimated to be 13,000 K from Tzvetkov et al. (2004) (J.-C. Loison 2018, private communication). This experiment was performed in the multilayer regime, and so the effect of the substrate will have a minimal impact. Experiments in the monolayer regime are needed in order to quantify the mixing of the deposited glycine molecule with the water ice substrate. Nevertheless, we used the higher binding energy obtained by the TPD experiment as the extreme case to see the effect of binding energy on the gas-phase abundance of glycine.

To simulate the physical conditions in hot cores, we have used the two-stage physical model: free-fall collapse, followed by a dynamically static warm-up by following Garrod (2013). The cold collapse phase starts at ${n}_{{{\rm{H}}}_{2}}=3\times {10}^{3}$ cm⁻³ and moves to the final post-collapse density of ${n}_{{{\rm{H}}}_{2}}=2\,\times {10}^{7}$ cm⁻³. The increase in visual extinction during collapse leads to the minimum dust-grain temperature of 8 K followed by a warm-up from 8 to 400 K; during this phase, the gas and dust temperatures are assumed to be well coupled and the gas density is fixed. We assumed fast and slow warm-up models described in Garrod (2013), whose timescales for the warm-up phases are 7.12 × 10⁴ yr and 1.43 × 10⁶ yr, respectively.

To develop a state-of-the-art chemical network for glycine, we have started from the gas-phase chemical network kida.uva.2014⁷ (Wakelam et al. 2015). It includes 489 species composed of 13 elements (H, He, C, N, O, Si, S, Fe, Na, Mg, Cl, P, F) linked with 7509 reactions. Garrod (2013) has adapted osu.2005 as an initial network to include the chemistry of glycine. We used similar elemental abundances to those from Garrod (2013), as listed here in Table 1. Also, we listed the binding energies used in our model in Table 2. The binding energies for some species were obtained by theoretical quantum chemical computation presented in Wakelam et al. (2017), but there are still some species whose binding energies were not measured or calculated. Our starting network includes various updates as compared to osu.2005 used in Garrod (2013). It considers the updates of HCN/HNC chemistry by Loison et al. (2014a), carbon chemistry by Loison et al. (2014b), branching ratios for reactions forming ${{\rm{C}}}_{n=2\mbox{--}10}^{(0,+)}$, ${{\rm{C}}}_{n=2\mbox{--}4}{{\rm{H}}}^{(0,+)}$, and C₃${{\rm{H}}}_{2}^{(0,+)}$ from Chabot et al. (2013), and also various new data sheets available in the KIDA database. Our network for surface reactions and gas-grain interactions is based on the one from Garrod et al. (2007) with several additional processes from Ruaud et al. (2015). We have added the following updates into our initial gas-grain chemical network:

• 1.  
  We have included the chemistry of glycine and other related COMs that are not available in the KIDA database from Garrod (2013).⁸ This includes mainly four radical-addition reactions on the grains for glycine formation:

  $$\begin{eqnarray}&&{\rm{s \mbox{-} NH}}{{\rm{CH}}}_{2}{\rm{COOH}}+{\rm{s \mbox{-} H}}\to {{\rm{s \mbox{-} NH}}}_{2}{{\rm{CH}}}_{2}{\rm{COOH}}\end{eqnarray} \tag{ 1 }$$

  $$\begin{eqnarray}&&{{\rm{s \mbox{-} NH}}}_{2}+{{\rm{s \mbox{-} CH}}}_{2}{\rm{COOH}}\to {{\rm{s \mbox{-} NH}}}_{2}{{\rm{CH}}}_{2}{\rm{COOH}}\end{eqnarray} \tag{ 2 }$$

  $$\begin{eqnarray}&&{{\rm{s \mbox{-} CH}}}_{2}{{\rm{NH}}}_{2}+{\rm{s \mbox{-} COOH}}\to {{\rm{s \mbox{-} NH}}}_{2}{{\rm{CH}}}_{2}{\rm{COOH}}\end{eqnarray} \tag{ 3 }$$

  $$\begin{eqnarray}&&{{\rm{s \mbox{-} NH}}}_{2}{{\rm{CH}}}_{2}{\rm{CO}}+{\rm{s \mbox{-} OH}}\to {{\rm{s \mbox{-} NH}}}_{2}{{\rm{CH}}}_{2}{\rm{COOH}}.\end{eqnarray} \tag{ 4 }$$

  These reactant radicals are mainly formed through either grain surface reactions or photodissociation of stable molecules. It also includes gas-phase destruction routes for glycine via UV photodissociation, either by the cosmic-ray-induced or standard interstellar radiation field or by ion–molecular reactions. The gas-phase reactions of CH₃COOH and protonated NH₂OH to form protonated glycine were also included from Garrod (2013), but the reaction rate coefficients of NH₂OH and its protonated species were updated from the theoretical calculation described in Barrientos et al. (2012).
• 2.  
  We have updated the chemistry of CH₂NH and CH₃NH₂ from Suzuki et al. (2016). These reactions are a series of hydrogenation processes to HCN on grains, as summarized in Table 3.
• 3.  
  We have included the glycine formation on the grains from Singh et al. (2013) via sequences of reactions with simple species, which are abundant in the ISM. They showed the following possible routes to form glycine via quantum chemical calculations:

  $$\begin{eqnarray}&&{{\rm{s \mbox{-} CH}}}_{2}+{{\rm{s \mbox{-} NH}}}_{2}\to {{\rm{s \mbox{-} CH}}}_{2}{{\rm{NH}}}_{2}\end{eqnarray} \tag{ 5 }$$

  $$\begin{eqnarray}&&{{\rm{s \mbox{-} CH}}}_{2}{{\rm{NH}}}_{2}+{\rm{s \mbox{-} CO}}\to {{\rm{s \mbox{-} NH}}}_{2}{{\rm{CH}}}_{2}{\rm{CO}}\end{eqnarray} \tag{ 6 }$$

  $$\begin{eqnarray}&&{{\rm{s \mbox{-} NH}}}_{2}{{\rm{CH}}}_{2}{\rm{CO}}+{\rm{s \mbox{-} OH}}\to {{\rm{s \mbox{-} NH}}}_{2}{{\rm{CH}}}_{2}{\rm{COOH}}.\end{eqnarray} \tag{ 4 }$$

  While reaction (2) was employed in Garrod (2013), Singh et al. (2013) suggested the new reaction sequences to finally form glycine involving CH₂CO, NH₂CH₂CO, and NH₂CHCO. We tested the importance of these reaction sequences for the first time. Singh et al. (2013) concluded that reactions (5) and (7) are barrierless reactions, while reaction (6) has an activation barrier of 7100 K. Another sequence of reactions starts from a combination of NH₂ and CH, as below:

  $$\begin{eqnarray}&&{{\rm{s \mbox{-} NH}}}_{2}+{\rm{s \mbox{-} CH}}\to {{\rm{s \mbox{-} NH}}}_{2}{\rm{CH}}\end{eqnarray} \tag{ 7 }$$

  $$\begin{eqnarray}&&{{\rm{s \mbox{-} NH}}}_{2}{\rm{CH}}+{\rm{s \mbox{-} CO}}\to {{\rm{s \mbox{-} NH}}}_{2}{\rm{CH}}{\rm{CO}}\end{eqnarray} \tag{ 8 }$$

  $$\begin{eqnarray}&&{{\rm{s \mbox{-} NH}}}_{2}{\rm{CH}}{\rm{CO}}+{\rm{s \mbox{-} OH}}\to {{\rm{s \mbox{-} NH}}}_{2}{\rm{CH}}{\rm{COOH}}\end{eqnarray} \tag{ 9 }$$

  $$\begin{eqnarray}&&{{\rm{s \mbox{-} NH}}}_{2}{\rm{CH}}{\rm{COOH}}+{\rm{s \mbox{-} H}}\to {{\rm{s \mbox{-} NH}}}_{2}{{\rm{CH}}}_{2}{\rm{COOH}}.\end{eqnarray} \tag{ 10 }$$

  There are no activation barriers for reaction (7), (8), and (9). However, reaction (10) possesses an activation barrier of as high as ∼37,000 K. The third path starts from CH₂ and CO:

  $$\begin{eqnarray}&&{{\rm{s \mbox{-} CH}}}_{2}+{\rm{s \mbox{-} CO}}\to {{\rm{s \mbox{-} CH}}}_{2}{\rm{CO}}\end{eqnarray} \tag{ 11 }$$

  $$\begin{eqnarray}&&{{\rm{s \mbox{-} CH}}}_{2}{\rm{CO}}+{\rm{s \mbox{-} OH}}\to {{\rm{s \mbox{-} CH}}}_{2}{\rm{COOH}}\end{eqnarray} \tag{ 12 }$$

  $$\begin{eqnarray}&&{{\rm{s \mbox{-} NH}}}_{2}+{{\rm{s \mbox{-} CH}}}_{2}{\rm{COOH}}\to {{\rm{s \mbox{-} NH}}}_{2}{{\rm{CH}}}_{2}{\rm{COOH}}.\end{eqnarray} \tag{ 2 }$$

  Reactions (11), (12), and (2) are barrierless reactions.
• 4.  
  Based on the latest high-level quantum chemical computation from Barrientos et al. (2012), we have also included the following reactions in the gas phase: NH₂OH₂⁺ + CH₃COOH with a barrier of 1150 K, NH₂OH⁺ + CH₃COOH with a barrier of 12,180 K, and NH₃OH⁺ + CH₃COOH with a barrier of 13,600 K. The formation of the positive ion of glycine, NH₂CH₂COOH₂⁺, will be followed by the dissociative recombination processes with an electron, to form glycine (∼2%) and other fragmented species (∼98%), such as NH₂, CH₂, and CH₂NH₂. All products and the reaction coefficients of these dissociative recombination processes were obtained from Garrod (2013), as summarized in Table 5 of Garrod (2013).

Table 3.  Hydrogenation Processes to HCN Discussed in Suzuki et al. (2016)

Reaction	EA (K)
N + CH3 $\longrightarrow$ CH2NH	EA = 0 K
NH + CH2 $\longrightarrow$ CH2NH	EA = 0 K
NH2 + CH $\longrightarrow$ CH2NH	EA = 0 K
HCN + H $\longrightarrow$ H2CN	EA = 3647 K
HCN + H $\longrightarrow$ HCNH	EA = 6440 K
H2CN + H $\longrightarrow$ CH2NH	EA = 0 K
HCNH + H $\longrightarrow$ CH2NH	EA = 0 K
CH2NH + H $\longrightarrow$ CH3NH	EA = 2134 K
CH2NH + H $\longrightarrow$ CH2NH2	EA = 3170 K
CH3NH + H $\longrightarrow$ CH3NH2	EA = 0 K
CH2NH2 + H $\longrightarrow$ CH3NH2	EA = 0 K

Note. The dust surface reactions related to CH₂NH and CH₃NH₂ are shown. E_A represents the value of the activation barrier. Since radical species are so reactive, radical–radical reactions would have no activation barriers. The activation barriers for HCN and CH₂NH were cited from the theoretical study by Woon (2002).

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Finally,⁹ we have nearly 4500 reactions on the grain surface linked with the 9500 reactions in gas phase.

With our model, we calculate the evolution of fractional abundances compared to the total proton density (hereafter presented as X). In Figures 1(a) and (b), the fractional ice abundances of H₂O, CO, CO₂, CH₃OH, NH₃, CH₄, and H₂CO are shown. CO₂ is mainly formed from CO and O on grains. On the other hand, CO is formed in the gas phase and accretes on grains. The formation processes of H₂O, NH₃, CH₄, H₂CO, and CH₃OH are hydrogenation processes to O, N, C and CO, respectively, on the grain surface. In our model, water ice starts to form at  1 × 10³ yr and shows its peak in between 2.005 × 10⁸ yr and 2.025 × 10⁸ yr. According to Ruaud et al. (2016), water ice reaches its peak in roughly 1 × 10⁶ yr. This timescale is much shorter than our timescale of 1 × 10⁸ yr. The main reason behind the different timescales is that Ruaud et al. (2016) assume a constant density of 1 × 10⁴ cm⁻³, whereas our model assumed the time evolution during the collapsing phase. In our model, water ice shows its peak after 2 × 10⁸ yr when density exceeds 1 × 10⁴ cm⁻³.

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The molecular percentages compared to H₂O ice are plotted in Figures 1(c) and (d). We compare these molecular percentages in our model with Boogert et al. (2015) in Table 4. It is to be noted that observed ices toward massive YSOs are known to be slightly to strongly affected by thermal processing (Boogert et al. 2015). As a result, comparison of abundances of key ice species at the end of collapse may not be directly relevant for simulations run under the collapse model, i.e., where the dust temperature is as low as 16 K at the initial visual extinction of Av = 2, and during collapse this falls to a minimum temperature of 8 K, whereas gas temperature remains constant at 10 K. The best-fit age is 2.026 × 10⁸ yr during the collapsing phase when the gas density gets high enough to form sufficient hydrogenated species via grain surface reactions.

3.2.1. Fast Warm-up Model

In Figure 2(a), we show the fractional abundances of glycine calculated with the fast warm-up model (hereafter we refer it as the Fast Model). The abundances in the gas phase, on grain surface, and in the grain mantle are presented using the red, blue, and black lines. Once the peak abundance of glycine is achieved on grains, it starts to be destroyed through the grain surface reactions before glycine is thermally desorbed.

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Figure 2(a) suggests that the peak abundance of glycine in the grain mantle is ∼1 × 10⁻⁹ at (4–5) × 10⁴ yr since the beginning of the warm-up phase. In other words, the formation of glycine is completed on the grains at that time. In Figure 3(a), the formation rates (cm⁻³ s⁻¹) of glycine on the grain surface are compared. The most dominant formation path on the grains is "s-CH₂NH₂ + s-COOH," whose peak is achieved when the temperature is 60–120 K at (4–5) × 10⁴ yr. In the peak of black lines, the HNCH₂COOH radical, which is produced via the destruction process of glycine, is used to reproduce glycine again via the hydrogenation process. Considering the fact that the abundance of glycine in the mantle achieved its peak at 5 × 10⁴ yr, when "s-CH₂NH₂ + s-COOH" is the most efficient, this process would be the major path on the grain surface.

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We plot the abundances of the glycine precursors CH₂NH₂ and COOH in Figures 2(b) and (c). The COOH radical is easily lost from the grain surface (blue dotted line) when the temperature is high (∼60 K). The formation rate of glycine on the grain surface via the "s-CH₂NH₂ + s-COOH" process decreases at ∼5 × 10⁴ yr, when the COOH radical is completely lost from the grain surface. The formation rates of CH₂NH₂ and COOH radicals are compared in Figures 3(b) and (c), in a similar manner to glycine.

In Figure 3(b), we compare the formation rates of CH₂NH₂ via "s-CH₂NH + s-H $\longrightarrow$ s-CH₂NH₂," "s-CH₃O + s-CH₃NH₂ $\longrightarrow$ s-CH₃OH + s-CH₂NH₂," and the sum of the formation rates via other processes with green, red, and black lines, respectively. The process of "s-CH₂NH + s-H $\longrightarrow$ s-CH₂NH₂" is efficient at the beginning of the warm-up phase, when the low temperature enabled the hydrogen atoms to be stored on grains. The other process, "s-CH₃O + s-CH₃NH₂ $\longrightarrow$ s-CH₃OH + s-CH₂NH₂," is efficient when the temperature is more than ∼60 K, so that heavier radicals can move on the grain surface by thermal hopping. Since glycine is formed on the grains before 5 × 10⁴ yr, the above two processes would be key origins of CH₂NH₂ radicals for the formation of glycine. The formation rates of the COOH radical are compared in Figure 3(c). The red line represents the formation rate of the COOH radical via the hydrogen subtraction process by H atoms from HCOOH: "s-H + s-HCOOH $\longrightarrow$ s-H₂ + s-COOH." The green and blue lines, respectively, represent the sums of the formation rates of the COOH radical via destruction processes by NH₂ and OH radicals, respectively, of carboxyl groups, HCOOH, CH₃OCOOH, and CH₂OHCOOH. The former process was dominant when the temperature is low, while the latter processes are important when the grain surface temperature gets warmer and radicals can move on the grains. Figure 2(c) shows that the peak of the COOH radical on the grain surface is achieved at 3 × 10⁴ yr, when the grain surface temperature is ∼30 K and the efficiency of "s-H + s-HCOOH $\longrightarrow$ s-H₂ + s-COOH" decreases. Despite the efficient conversion process of HCOOH to COOH on grains at this age, the reverse hydrogenation process of "s-H + s-COOH $\longrightarrow$ s-HCOOH" suppresses the abundance of the COOH radical on the grains. Once hydrogen atoms are liberated from grains at 3 × 10⁴ yr, the hydrogenation rate to convert the COOH radical to HCOOH decreases. As a result, the destruction processes of carboxyl groups by NH₂ and OH radicals increase COOH radicals on the grain surface. The above formation process of glycine on grains agrees with Garrod (2013).

Despite the agreement on the formation process of glycine on the grains with Garrod (2013), our chemical model suggests that the major formation process of gas-phase glycine is gas-phase reactions rather than the thermal evaporation of grains with its binding energy of 13,000 K, contrary to Garrod (2013). In Figure 3(d), we plot the subtraction of the accretion rate of gas-phase glycine from the evaporation rate of glycine on grains. The accretion rate always overwhelms the evaporation rate, suggesting that the thermal evaporation is not efficient. The comparison of the gas-phase formation rates of glycine in Figure 3(e) shows that "NH₂CH₂COOH₂⁺ + e⁻" is the major formation path to glycine at ∼5 × 10⁵ yr since the beginning of the warm-up. NH₂CH₂COOH₂⁺ is produced via the reaction of CH₃COOH and NH₂OH₂⁺. NH₂OH₂⁺ is formed from NH₂OH with the reactions of positive ions such as ${{\rm{H}}}_{3}^{+}$ and H₃O⁺, as shown in Figure 3(f). In our model, both CH₃COOH and NH₂OH are efficiently built on grains (Figures 3(g) and (h)). The formation of NH₂OH is almost completed during the collapsing phase. Once HNO is formed in the gas phase, it accretes on grains and was converted to NH₂OH via the hydrogenation processes. CH₃COOH is built on grains from CH₃CO and OH radicals. We conclude that the series of above reactions are the most essential process to form gas-phase glycine in this model. We note that the predicted abundances and the formation mechanisms discussed here are subject to the binding energies, reaction rates, and physical evolution of the core and should be updated based on the latest progress of these studies. Especially, the precise value of binding energy of glycine should be explored in detail in future works as the essential parameter to change the predicted glycine abundances, major formation processes, and key precursors.

3.2.2. Slow Warm-up Model

In this subsection, we will present our results under the slow warm-up model, where the timescale of warm-up is 1.43 × 10⁶. We show the simulated abundances and the formation route to glycine in Figures 4 and 5(a) in the same manner as the Fast Model. In this model (referred to as the Slow Model), the gas-phase peak abundance of glycine is much smaller than those on the grain surface and in the grain mantle. The peak abundance of glycine in the gas phase is lower than in the fast warm-up model, due to the destruction process on grains of glycine by radicals such as OH and NH before thermal evaporation.

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In Figure 5(a), the formation rates (cm⁻³ s⁻¹) of glycine on grain surface are compared. The contributions of other reactions are mainly due to "s-H + s-NHCH₂COOH," where NHCH₂COOH is formed via destruction processes of glycine. However, similar to the fast warm-up model, the most dominant process is "s-CH₂NH₂ + s-COOH," and its peak is achieved when the temperature is between 60 and 120 K at (8–10) × 10⁵ yr.

The abundances and the formation rates of the glycine precursors on grains, CH₂NH₂ and COOH, are compared in Figures 4 and 5(b) and (c) in the same way as the fast warm-up model. CH₂NH₂ is produced via the destruction processes of CH₃NH₂ by NH, OH, and CH₃O radicals. The formation process of COOH was the destruction of HCOOH, which is consistent with the fast warm-up model, while the destruction by UV photons is more important than the fast warm-up model owing to the longer timescale. For the overall trend, the formation process of glycine on the grains does not depend on the warm-up speed.

However, similar to the Fast Model, the fact that the accretion rate of glycine is higher than its evaporation rate (Figure 5(d)) suggests an efficient formation of this molecule in the gas phase. In the Slow Model, glycine is formed from the reaction of NH₂OH₂⁺ with CH₃COOH (Figure 5(e)). NH₂OH₂⁺ is formed from the gas-phase reactions of NH₂OH with positive ions, mainly H₃O⁺ (Figure 5(f)). Both NH₂OH and CH₃COOH thermally evaporate during the warm-up phase (Figures 5(g) and (h)).

We report in Table 5 the gas-phase glycine abundance computed with our model and the one from Garrod (2013; Table 8). Similarly, the peak abundances (in the gas, in the surface, and in the mantle) of glycine precursors, NH₂OH, CH₂NH, CH₃NH₂, HCOOH, and CH₃COOH, in Garrod (2013) are shown in Tables 6 and 7, together with the temperature corresponding to the peak. We note that we have only gas-phase abundances for the Garrod (2013) results. It is notable that the peak abundance of glycine was decreased by about a factor of 100 in our model as compared to Garrod. This discrepancy is due to the high desorption energy of glycine in our model, determined by the result of the TPD experiment by Tzvetkov et al. (2004). With the binding energy of 13,000 K, glycine is destroyed through grain surface reactions with NH or CH₃O radicals before it fully evaporates. The dramatic increase of NH₂OH and CH₃NH₂ in our model is due to the inclusion of a series of hydrogenation processes to NO and HCN, respectively. The increase of CH₂NH would be due to the update of gas-phase chemistry in kida.uva.2014, which is described in detail in Suzuki et al. (2016). In the Slow Model, CH₃COOH is depleted compared to Garrod (2013). Considering that this depletion is not seen in the Fast Model, the depletion is due to the longer timescale of the collapsing phase and the different amount of radical species to destroy CH₃COOH before its evaporation.

If the binding energy is 13,000 K, formation processes of glycine in the gas phase are more important than thermal evaporation from grains. The higher accretion rate than the evaporation rate shows the gas-phase origin of glycine (Figure 5(d)). In this case, glycine is formed via "NH₂CH₂COOH₂⁺ + e⁻," and the precursors are CH₃COOH and NH₂OH. These species are built on grains and then thermally evaporate into gas phase (Figures 4(d) and (e)).

We compare the abundances of glycine in the gas phase in Figure 6, changing the desorption energies of glycine to be 10,100 K, corresponding to the one in Garrod (2013). If we employ the same desorption energy as Garrod (2013) in the Fast Model (b), we obtain a peak abundance of gas-phase glycine of 3.1 × 10⁻¹¹ for the fast warm-up model. This value agrees with Garrod (2013), where the peak abundance of glycine was reported to be 8.4 × 10⁻¹¹. These results clearly suggest that the discrepancy of the formation process of gas-phase glycine is due to the different binding energy of glycine, being the essential parameter to affect the abundance and the detectability of gas-phase glycine. With the binding energy of 10,100 K for glycine, the gas-phase abundance is mostly due to thermal evaporation.

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These results strongly suggest that the binding energy of glycine is a key parameter that affects the abundance of gas-phase glycine. The further study of TPD experiments under the monolayer regime is required to improve our understanding further.

In this subsection, we will discuss whether the actual abundances of precursors of glycine can be explained with our Fast Model. Favre et al. (2017) detected CH₃COOH toward the edge of the Orion KL Hot Core and reported two velocity components with CH₃COOH column densities of 1.2 × 10¹⁶ cm⁻² and 3.3 × 10¹⁵ cm⁻². Since the peak position of CH₃COOH is close to the source called HKKH7 reported in Hirota et al. (2015), we use the hydrogen column density of 1.1 ×10²⁵ cm⁻² based on the estimation by Hirota et al. (2015). Then, as summarized in Table 8, the fractional abundances of CH₃COOH in these components are, respectively, 1.0 × 10⁻⁹ and 3.0 × 10⁻¹⁰. This value is close to (0.9–7) × 10⁻¹⁰, corresponding to that of Sgr B2 (Mehringer et al. 1997). With our peak fractional abundance of 1.0 × 10⁻¹⁰, these values can be explained within a factor of 10. On the other hand, for NH₂OH, the upper limits of NH₂OH toward Orion KL and Sgr B2 are, respectively, 3 × 10⁻¹¹ and 8 × 10⁻¹² (Pulliam et al. 2012). These upper limits of NH₂OH suggest that we strongly overestimated the abundance of NH₂OH. Recently, Jonusas & Krim (2016) have experimentally shown that heating of NH₂OH–H₂O ices leads to a decomposition of NH₂OH into HNO, NH₃, and O₂ at 120 K, and it is possible that the lack of this effect may have led to strong overestimation of NH₂OH in our model. Since glycine is formed in the gas phase using the positive ion of NH₂OH, the gas-phase abundance of glycine would be overestimated and gives us only the upper limit. This result also suggests the importance of thermal decomposition processes of not only NH₂OH but also other species for the high-temperature chemistry.

On the grain surface, the important precursors are CH₂NH, CH₃NH₂, and HCOOH. As Suzuki et al. (2016) claimed that gas-phase CH₂NH would originate from the gas-phase reaction of "NH + CH₃," the gas-phase abundance of CH₂NH does not represent the grain surface origin of CH₂NH. Therefore, we will focus on HCOOH and CH₃NH₂ abundances in the gas phase for the benchmark of our calculation. Liu et al. (2002) reported the fractional abundance of HCOOH to be ∼3 × 10⁻⁹ toward Orion KL, which agrees well with our peak value of 3.4 × 10⁻⁹. However, our HCOOH abundance is higher than that of Sgr B2 (Ikeda et al. 2001), probably because the physical condition of Sgr B2 is different from our model. Pagani et al. (2017) reported the actual observation of CH₃NH₂ toward Orion KL to be 1 × 10¹⁶ cm⁻², corresponding to the fractional abundance of 1 × 10⁻⁹ assuming a hydrogen column density of 1.1 × 10²⁵ cm⁻² (Hirota et al. 2015). Halfen et al. (2013) reported similar CH₃NH₂ abundance of 1.7 × 10⁻⁹ toward Sgr B2. Since our peak abundance of CH₃NH₂ is 7.6 × 10⁻⁶, we overestimated the abundance of CH₃NH₂ in the gas phase. This disagreement may be due to the fact that (1) hydrogenation processes to HCN and CH₂NH are not as efficient as we assume, and/or (2) the actual source age is ∼10⁵ yr after the completion of the warm-up and gas-phase CH₃NH₂ has already been destroyed in Orion KL. We will also assess the latter possibility in the subsequent section.

The first detection of glycine is one of the biggest challenges in the field of radio astronomy. In this subsection, we will discuss the future detectability of glycine.

In Suzuki et al. (2017), we performed the chemical modeling study for high-mass star-forming regions, where we assumed that the temperature gradient inside hot cores would be represented by hot and warm temperature. While the temperature in the inner part of the hot core was set to be 200 K, the temperature of the outer region of the hot core was taken as a free parameter. We looked for the best combination of parameters in our modeling to explain the observed abundance of COMs, changing the age of the core, the temperature of warm region, and the volume ratio of the hot and warm region inside the hot core, using "the degree of proximity" from Wakelam et al. (2006) as a criterion to compare the modeling results with observed abundances. Through the comparison, we suggested that the ages of G10.47+0.03 and NGC 6334F were, respectively, 8 × 10⁵ yr and 6.5 × 10⁵ yr after the birth of the star, where chemical compositions were altered via the gas-phase reactions after the thermal evaporation. Hence, these ages would be reasonable to predict the abundances of glycine or other COMs in G10.47+0.03 and NGC 6334F, although Garrod (2013) stopped the chemical evolution just after the completion of the warm-up phase. In this subsection, we will simulate the chemical evolution up to 1 × 10⁶ yr to compare with Suzuki et al. (2017), fixing the temperature and the density once the warm-up phase is completed.

We will use the Fast Model with the binding energy of 13,000 K for glycine. In Figure 7 we plotted the time evolution of the gas-phase abundance of glycine with the peak temperature of 400 K, along with the gas-phase abundances of CH₂NH, CH₃NH₂, NH₂OH, and CH₃COOH. The temperature of 400 K corresponds to the environment of the closer region to the central star and is more susceptible to stellar radiation than the 200 K region in Suzuki et al. (2017). We note that the gas-phase abundance of glycine is higher than that of Table 6, since the longer timescale enabled the gas-phase reaction of "NH₂OH₂⁺ + CH₃COOH" to form protonated glycine, followed by the formation of glycine through the dissociative recombination with an electron. We plotted the time evolutions of glycine, CH₃NH₂, CH₂NH, NH₂OH, and CH₃COOH in Figure 7. Figure 7 implies that the abundances of glycine, CH₃NH₂, NH₂OH, and CH₃COOH will decrease over time owing to the destruction processes by positive ions and radicals. The gas-phase abundance of glycine between 6.5 × 10⁵ yr and 8 × 10⁵ yr was ∼10⁻¹⁴, lowered by one order of magnitude compared to its peak abundance.

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Our modeling results suggest that the gas-phase abundance of glycine will be lowered by almost one order of magnitude after 5 × 10⁵ yr since the beginning of the warm-up. Hence, the very early phase of the hot cores, where gas-phase destruction processes are less efficient, would be the ideal target sources to observe gas-phase glycine. We show the time evolutions of glycine and its precursors, NH₂OH and CH₃COOH, in Figure 7. Figure 7 tells us that NH₂OH and CH₃COOH can be used as the direct precursor to search for glycine-rich sources since they contribute directly in chemical kinetics of glycine in the gas phase and also follow similar evolutionary paths. We note that the number of sources of CH₃COOH is still limited. NH₂OH has not been detected in the ISM yet. Future survey observations of these species in the hot components will be helpful to constrain our chemical modeling. In addition, molecules built on grain surface, such as CH₃NH₂, can be used as the indicator of the early-phase hot core and hence potentially glycine-rich sources. They decrease with time via the destruction processes in the gas phase. By contrast, molecules originating in the gas-phase reactions, such as CH₂NH (Suzuki et al. 2016), do not show this trend. These species would also give us useful information regarding the age of the hot cores.

The comets are believed to be formed via the aggregation of interstellar dust particles within the protoplanetary disks, and the link between interstellar chemistry and the chemical compositions of comet is an interesting problem. The Rosetta mission reported many organic compounds in the coma of comet 67P/Churyumov–Gerasimenko. In the past, Biver et al. (2015) found a good agreement in cometary species and those in warm molecular clouds. This motivated us to compare our model predictions for glycine with the recent detection in comet 67P.

The recent detection of glycine by the Rosetta mission in 67P/Churyumov–Gerasimenko (Altwegg et al. 2016) implies that glycine would possibly be inherited from the parental cloud of our solar system. Altwegg et al. (2016) reported the detections of glycine in several positions, with its highest abundance ratio to water, "Glycine/H₂O," of $1.6\times {10}^{-3}$. Our fast and slow warm-up models developed for the high- and low-mass stars would give us some insights regarding the pristine chemical compositions preserved in the comets.

Comet are believed to have been formed via the accretion of interstellar dust particles. Considering that the large part of molecules would be frozen in the grain mantle rather than on the grain surface, the chemical composition of the grain mantle would be inherited by the cometary materials. We plotted the abundance ratio in the grain mantle of "Glycine/H₂O" in Figure 8 with green lines. While the evaporation rate for glycine is very low below 300 K owing to its high binding energy, H₂O is quickly lost compared to glycine. As a result, the ratio of "glycine/H₂O" has increased, and "glycine/H₂O < $1.6\times {10}^{-3}$" is achieved when the temperature is less than 127 K. At that moment, the fractional abundances of glycine and water in the solid phase are, respectively, $9.4\times {10}^{-10}$ and $6.9\times {10}^{-7}$.

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3.7.1. The Background of Suprathermal H*

Muñoz Caro et al. (2002) conducted laboratory experiments and confirmed 16 amino acids, including glycine, after UV irradiation on ice containing H₂O, CH₃OH, NH₃, CO, and CO₂. Their experiments suggested that the formation of glycine is possible without HCN. However, the detailed formation mechanism was unclear at that time. To deepen the understanding of formation path to glycine in this scheme, Holtom et al. (2005) conducted experiments assuming a cosmic-ray-inducing environment on the interstellar ice analog containing CH₃NH₂ and CO₂, under the assumption that CH₃NH₂ would have formed from CH₃ and NH₂ radicals during the experiments by Muñoz Caro et al. (2002). As a result, they reported the detection of glycine. They claimed that this process starts from photo cleavages of C–H and N–H bonds in CH₃NH₂ and creates CH₂NH₂ and H. Although a grain surface process of "s-CO₂ + s-H $\longrightarrow$ s-COOH" is not likely to occur owing to its high activation barrier of 65.6 kJ mol⁻¹ (∼7900 K; Zhu et al. 2001), the nascent H atom by strong UV photons would have extra energy to overcome this barrier (Holtom et al. 2005; Lee et al. 2009). The reaction "s-CH₂NH₂ + s-COOH $\longrightarrow$ s-NH₂CH₂COOH" has no entrance barrier (Holtom et al. 2005). Furthermore, Lee et al. (2009) reported the formation of glycine from CH₃NH₂ and CO₂ on interstellar ice analog films under UV irradiation at 7.3–10 eV, corresponding to the effective energy range for interstellar UV radiation. We note that Garrod (2013) also claimed the importance of the reaction of "s-CH₂NH₂ + s-COOH $\longrightarrow$ s-NH₂CH₂COOH." However, he assumed that COOH was formed from the subtraction of an H atom from HCOOH by radicals such as OH, rather than the hydrogenation process to CO₂.

Two sources of UV photons would be available to form H* in actual star-forming regions. The first possibility would be the blackbody emission from the central star. However, we deduce that this process is important in only a limited region through the following calculation with the assumption of a spherical symmetric sphere.

It is known that Av is proportional to the hydrogen column density: N[H] (cm⁻²) = 2.21 × 10²¹ Av (mag) (Güver & Özel 2009). According to the density profile based on the observations and theoretical studies of UCHII regions by Nomura & Millar (2004), the hydrogen atom number densities (n_H) are not so different within 0.1 pc. With a peak density of 1 × 10⁷ cm⁻³, we are able to calculate Av by using the distance from the star "r" as follows:

$$\begin{eqnarray}&&{Av}=1.4\times {10}^{4}\times r(\mathrm{pc}).\end{eqnarray} \tag{ 13 }$$

In NAUTILUS, the photodissociation rates are presented by the formula below with Av:

$$\begin{eqnarray}&&k=A\exp (-{CAv})\,{{\rm{s}}}^{-1}.\end{eqnarray} \tag{ 14 }$$

The coefficients A and C can be theoretically and/or experimentally obtained. In this case, the photodissociation rate will decrease by a factor of ∼8.3 × 10⁻⁷ by every 0.01 pc. Since the UV photons from the star would be available only in a limited region, we do not employ the star as the source of UV photons.

The other possibility for the source of UV photons is related to the cosmic rays. Prasad & Tarafdar (1983) suggested that the cosmic rays are an important source of UV flux in dense regions with large Av, where interstellar UV photons cannot penetrate. These mechanisms are included in kida.uva.2014. For the modeling of H*, we simply assumed that H* is created by the dissociation process of any H-bearing species by a UV field by the Prasad–Tarafdar mechanism instead of H.

For instance,

$$\begin{eqnarray*}&&{\rm{s}} \mbox{-} {\mathrm{CH}}_{3}{\mathrm{NH}}_{2}+h\nu \longrightarrow {\rm{s}} \mbox{-} {\rm{H}}* +{\rm{s}} \mbox{-} {\mathrm{CH}}_{2}{\mathrm{NH}}_{2},\mathrm{or}\\ &&{\rm{s}} \mbox{-} {\mathrm{NH}}_{3}+h\nu \longrightarrow {\rm{s}} \mbox{-} {\rm{H}}* +{\rm{s}} \mbox{-} {\mathrm{NH}}_{2}.\end{eqnarray*}$$

The activation energy for the grain surface reaction, "s-CO₂ + s-H $\longrightarrow$ s-COOH," is ∼7900 K (0.68 eV). When UV photons with their energy of above 5 eV destroy CH₃NH₂, ∼4.3 eV is used to dissociate the C–H bond, and extra energy is given to the dissociated hydrogen atoms that can sufficiently overcome the potential barrier associated with the "s-CO₂ + s-H" process. Since all grain surface reactions in our model have lower activation barriers than "s-CO₂ + s-H $\longrightarrow$ s-COOH" (E_A ∼ 7900 K), H* would be able to overcome any activation barriers associated with grain surface reactions in our model. Therefore, we develop a new model referred to as the "Fast + H* Model," where H* can lead to any hydrogenation process by penetrating activation barriers (E_A = 0 K). For instance, not only "s-CO₂ + s-H* $\longrightarrow$ s-COOH" but also other grain surface hydrogenation processes such as "s-CO + s-H* $\longrightarrow$ s-HCO" are treated as barrierless reactions. We present a diagram of formation paths of H* and COOH in the above scenario in Figure 9.

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For simplification, other chemical properties for H* were assumed to be the same as the usual H atoms. It was assumed that H* acts as a usual H atom in the gas-phase chemistry after the evaporation process.

The inclusion of H* changes the reaction rates of hydrogenation processes dramatically. Recall that reaction rates on grains are proportional to ${\kappa }_{{ij}}$, as is given in the below formula using activation energy E_A:

$$\begin{eqnarray}&&{\kappa }_{{ij}}=\exp {(-2(a/{\hslash })(2\mu {E}_{A})}^{1/2}),\end{eqnarray} \tag{ 15 }$$

where "a" is the rectangle barrier of thickness (1 Å) and μ is the reduced mass. ${\kappa }_{{ij}}$ is 2.7 × 10⁻¹⁶ for the usual hydrogenation process to CO₂ (s-H + s-CO₂ $\longrightarrow$ s-COOH), with its activation energy of ∼7900 K. With such a low reaction rate, the usual hydrogenation process to CO₂ would be negligible even if CO₂ is abundant. However, ${\kappa }_{{ij}}$ becomes unity for hydrogenation processes of H*, where E_A = 0. The conversion of CO₂ to COOH by the hydrogenation process of H* may strongly change the abundances of COOH on grains.

We also added the conversion processes of H* to usual hydrogen atoms:

$$\begin{eqnarray*}&&{\rm{s}} \mbox{-} {\rm{H}}* \,+\,{\rm{s}} \mbox{-} {\rm{H}}* \longrightarrow {\rm{s}} \mbox{-} {{\rm{H}}}_{2},\\ &&{\rm{s}} \mbox{-} {{\rm{H}}}_{2}+{\rm{s}} \mbox{-} {\rm{H}}* \longrightarrow {\rm{s}} \mbox{-} {{\rm{H}}}_{2}+{\rm{s}} \mbox{-} {\rm{H}},\mathrm{and}\\ &&{\rm{s}} \mbox{-} {\rm{H}}+{\rm{s}} \mbox{-} {\rm{H}}* \longrightarrow {\rm{s}} \mbox{-} {{\rm{H}}}_{2}.\end{eqnarray*}$$

Finally, we note that we neglect the thermalization process of H* in our model. According to the theoretical study of Andersson & van Dishoeck (2008), the H atom following the UV photodissociation can move tens of angstroms before the thermalization due to interactions with H₂O ice. Although the timescale of thermalization and chemical reaction of H* is not well known, it is thought that the timescale chemical reaction should be longer than this thermalization process (Cuppen & Walsh 2017). Therefore, our model may overestimate the effect of H*, and the impact of thermalization process for the chemical evolution has to be deeply investigated in the following studies.

3.7.2. Impacts of Suprathermal Hydrogen Atoms on the Abundance of Glycine

3.7.3. Implication of the H* Chemistry on the Other COMs

We showed the chemical composition for the selected COMs in Figure 10, for the Fast Model and the "Fast + H* Model." The time of zero corresponds to the beginning of the warm-up phase. The solid and dotted lines, respectively, represent the abundances in gas phase and on grains (surface and mantle).

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Figure 10 suggests that the suprathermal hydrogen atoms do not change the abundance of simple molecules, such as CH₄, CO, CO₂, and H₂O, and COMs, such as CH₃OH, HCOOCH₃, CH₃OH, and CH₃NH₂. However, with the reaction of "s-CO₂ + s-H*," the abundance of the COOH radical has increased by more than a factor of 10. As a result, the abundance of molecules containing the carboxyl group (i.e., HCOOH, CH₃COOH, CH₃CH₂COOH, NH₂CH₂COOH) has increased. We note that the molecules discussed in Suzuki et al. (2017), CH₃OH, HCOOCH₃, CH₃OCH₃, (CH₃)₂CO, NH₂CHO, NH₂CHO, CH₂CHCN, and CH₂NH, are not sensitive to the addition of H*, suggesting that the "Fast + H* Model" is able to reproduce the observed abundance in the same manner as our previous model (Suzuki et al. 2017). We summarized the peak abundances of important species in Table 9, which showed the trend that the abundances of carboxyl groups were enhanced compared with the other species. The species with the carboxyl group were mainly formed on grains via the radical–radical reactions with the COOH radical. We suggest that the observations of these species toward various star-forming regions would be a key to discussing the importance of suprathermal hydrogen atoms.

Table 9.  The Peaks of the Simulated Abundances for the Fast Model and the "Fast + H* Model"

	Fast Model						Fast + H* Model
	Gas		Surface		Mantle		Gas		Surface		Mantle
Species	X	T (K)	X	T (K)	X	T (K)	X	T (K)	X	T (K)	X	T (K)
CH3	2.5 (−6)	400	7.5 (−13)	10	4.0 (−11)	10	2.5 (−6)	400	1.4 (−12)	10	7.8 (−11)	10
NH2	4.1 (−9)	400	7.4 (−13)	10	3.6 (−11)	10	4.0 (−9)	400	7.0 (−13)	10	3.4 (−11)	10
CN	3.2 (−10)	90	1.5 (−16)	44	9.2 (−15)	10	2.1 (−10)	90	2.3 (−16)	33	1.8 (−14)	44
OCH3	2.4 (−9)	125	1.5 (−8)	10	9.2 (−7)	10	1.6 (−9)	125	1.8 (−8)	10	1.1 (−6)	10
CH2OH	5.1 (−11)	132	4.8 (−9)	10	2.9 (−7)	10	3.4 (−11)	132	6.0 (−9)	10	3.7 (−7)	10
HOCO	5.2 (−11)	167	4.2 (−11)	19	2.6 (−9)	19	6.0 (−10)	177	3.6 (−10)	10	2.2 (−8)	10
CH2NH2	1.7 (−10)	132	4.6 (−10)	10	2.8 (−8)	10	9.9 (−11)	132	5.4 (−10)	10	3.3 (−8)	10
H2O	2.4 (−4)	400	5.3 (−6)	125	2.3 (−4)	103	2.4 (−4)	400	5.3 (−6)	125	2.3 (−4)	103
CO	4.4 (−6)	400	7.3 (−9)	19	4.5 (−7)	19	4.9 (−6)	400	8.2 (−9)	19	5.1 (−7)	19
H2CO	1.7 (−6)	224	3.9 (−8)	10	2.4 (−6)	10	2.3 (−6)	207	4.7 (−8)	10	2.9 (−6)	10
CH3OH	7.3 (−5)	125	1.3 (−6)	33	7.7 (−5)	11	7.3 (−5)	125	1.4 (−6)	33	7.6 (−5)	11
CO2	1.0 (−6)	132	1.7 (−8)	59	9.7 (−7)	59	1.4 (−6)	132	2.4 (−8)	59	1.2 (−6)	44
O2	5.3 (−9)	68	3.0 (−13)	10	1.8 (−11)	10	9.1 (−9)	68	1.1 (−12)	10	6.9 (−11)	10
O3	7.9 (−11)	68	4.4 (−12)	25	2.7 (−10)	25	3.3 (−10)	68	1.3 (−11)	33	8.1(−10)	33
HCN	3.9 (−7)	400	4.3 (−9)	73	2.4 (−7)	73	3.4 (−7)	400	2.7 (−9)	73	2.1 (−7)	68
HNC	4.9 (−8)	97	3.2 (−10)	84	1.8 (−8)	84	3.3 (−8)	90	3.2 (−10)	84	3.1 (−8)	84
CH4	3.5 (−5)	33	5.7 (−7)	19	3.5 (−5)	19	3.4 (−5)	33	5.7 (−7)	19	3.5 (−5)	19
NH3	5.7 (−5)	141	1.2 (−6)	117	5.6 (−5)	14	5.4 (−5)	141	1.1 (−6)	117	5.3 (−5)	14
C2H2	1.3 (−7)	400	7.9 (−10)	59	4.4 (−8)	59	1.3 (−7)	400	9.8 (−10)	59	4.2 (−8)	44
C2H4	5.0 (−7)	400	3.2 (−9)	59	1.8 (−7)	59	5.8 (−7)	400	3.9 (−9)	59	1.7 (−7)	44
C2H6	1.8 (−7)	59	5.0 (−9)	14	3.1 (−7)	14	1.6 (−7)	59	5.0 (−9)	14	3.1 (−7)	14
HNO	5.8 (−9)	79	1.0 (−9)	10	6.3 (−8)	10	3.9 (−9)	73	1.6 (−9)	10	1.0 (−7)	10
NO	3.6 (−8)	125	6.0 (−10)	10	3.7 (−8)	10	3.3 (−8)	125	9.6 (−10)	10	5.9 (−8)	10
OCN	1.9 (−10)	103	7.1 (−14)	10	4.4 (−12)	10	1.9 (−10)	90	6.2 (−13)	10	3.7 (−11)	10
SO	1.1 (−8)	73	2.9 (−10)	33	1.6 (−8)	33	1.4 (−8)	73	3.9 (−10)	33	2.2 (−8)	33
SO2	3.7 (−9)	400	6.5 (−12)	33	3.6 (−10)	33	4.2 (−9)	400	1.6 (−11)	33	7.3 (−10)	33
NH2CHO	1.8 (−6)	400	4.0 (−10)	136	2.3 (−8)	19	1.9 (−6)	400	4.5 (−10)	19	2.8 (−8)	19
NH2OH	1.5 (−6)	158	6.8 (−7)	132	1.5 (−6)	25	2.0 (−6)	158	8.9 (−7)	136	2.1 (−6)	33
CH3CN	6.2 (−9)	307	8.9 (−11)	97	4.9 (−9)	97	4.7 (−9)	307	7.0 (−11)	97	4.1 (−9)	97
CH2NH	5.6 (−7)	400	1.0 (−9)	132	5.3 (−8)	10	5.7 (−7)	400	2.1 (−9)	132	6.3 (−8)	90
CH3NH2	7.4 (−6)	149	3.0 (−6)	132	8.3 (−6)	11	8.0 (−6)	149	3.0 (−6)	132	8.9 (−6)	11
NH2CH2COOH	4.6 (−14)	400	1.2 (−9)	163	1.6 (−9)	117	5.5 (−9)	293	1.4 (−8)	167	1.4 (−7)	117
HC3N	2.6 (−9)	400	1.1 (−11)	97	5.8 (−10)	97	1.5 (−9)	400	4.0 (−12)	90	1.7 (−10)	90
CH2CHCN	3.9 (−9)	90	3.9 (−10)	97	2.2 (−8)	97	4.8 (−9)	400	4.3 (−10)	97	2.9 (−8)	97
CH3CH2CN	4.8 (−8)	125	6.3 (−10)	103	3.5 (−8)	103	2.5 (−8)	125	2.8 (−10)	33	1.6 (−8)	103
CH3CH2OH	1.2 (−7)	136	2.4 (−9)	117	1.2 (−7)	44	1.5 (−7)	136	3.0 (−9)	117	2.5 (−7)	68
CH3CHO	4.2 (−9)	400	1.8 (−11)	10	1.1 (−9)	10	4.3 (−9)	400	2.6 (−11)	10	1.6 (−9)	10
CH3COCH3	2.8 (−9)	400	3.5 (−13)	68	2.0 (−11)	59	2.4 (−9)	400	5.9 (−13)	68	1.2 (−8)	68
HCOOCH3	8.7 (−10)	110	4.9 (−9)	10	3.0 (−7)	14	7.4 (−10)	110	5.2 (−9)	19	3.2 (−7)	19
CH3OCH3	1.5 (−7)	400	1.8 (−9)	44	1.0 (−7)	44	1.7 (−7)	400	2.4 (−9)	68	2.3 (−7)	68
(CH2OH)2	5.4 (−14)	248	4.1 (−14)	192	6.0 (−14)	79	2.6 (−13)	248	2.0 (−13)	192	4.7 (−14)	73
HCOOH	3.4 (−9)	314	3.6 (−10)	10	2.2 (−8)	10	2.2 (−8)	286	9.0 (−10)	14	5.5 (−8)	14
CH3COOH	1.0 (−10)	84	1.1 (−13)	25	6.7 (−12)	25	8.5 (−11)	84	3.9 (−13)	25	1.2 (−11)	25
NH2COOH	2.1 (−11)	207	3.9 (−11)	163	5.8 (−11)	117	1.4 (−10)	207	1.2 (−10)	163	1.9 (−10)	117
CH3OCOOH	1.0 (−9)	182	6.6 (−10)	145	1.1 (−9)	44	1.3 (−8)	182	8.9 (−9)	145	5.4 (−8)	68
CH2OHCOOH	9.1 (−10)	242	7.0 (−10)	192	9.8 (−10)	44	1.2 (−8)	242	9.5 (−9)	192	7.3 (−8)	68
C2H5COOH	5.4 (−10)	207	9.5 (−10)	163	1.4 (−9)	103	2.3 (−8)	213	2.0 (−8)	163	1.7 (−7)	84

Note. The peak of the fractional abundances and the temperature of grains at that moment were summarized. The notation of "a (b)" represents a × 10^b.

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3.7.4. Dependence of the Binding Energy of H*

One of the essential parameters to determine the significance of H* would be the binding energy. Lowering binding energy will allow the H* atom to diffuse rapidly on grains, while the timescale to reside on the grain surface will become short. In the above section, we assumed that the binding energy of H* is the same as that of the usual H atom; however, its extra kinetic energy would increase the opportunity for H* to move, making the binding energy much smaller than usual the H atom. Despite the importance of this, the value of the best binding energy of H* is not known. It has to be carefully determined considering the frequency dependency of UV flux inside the cores. In this subsection, we show the effect of binding energy on the abundance of glycine.

In Figure 11, we showed the abundances of glycine in the gas phase and on grains (surface and mantle), using the solid and dotted lines, respectively. In this modeling, we used the different binding energies of 650, 300, and 100 K, and its diffusion energy was given as 40% of its binding energy. With the binding energy of 650 K, the peak abundance of glycine on grains is 2.0 × 10⁻⁸, while they are 1.3 × 10⁻⁸ and 6.5 × 10⁻⁹, respectively, with the binding energies of 300 and 100 K. The peak gas-phase abundances of glycine are 5.5 × 10⁻⁹, 1.5 × 10⁻⁹, and 2.1 × 10⁻¹¹, respectively, with the binding energies of 650, 300, and 100 K. This result suggests that the smaller binding energy of H* will decrease the abundance of glycine on grains owing to more efficient thermal evaporation than the usual H atom. Detailed studies regarding kinetics of H* would be required to further promote the knowledge of the formation of glycine and to predict its reliable abundance by chemical modeling studies.

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