8.3 Implications for Interstellar Chemistry
8.3.3 Discussion
The diffusion rates shown in Table 8.6 indicate that the simpler photolysis products will dominate grain surface chemistry at low temperatures. H will clearly be the most mobile species on grain surfaces, and so it is likely to immediately react with any radical produced during photolysis at 10 K. This mechanism indicates a buildup of simple species such as CH3OH, H2O, CH4, NH3, and H2CO on cold grain surfaces, a conclusion
Table 8.6: Diffusion barriers and rates for reactive surface species.
ED ED Rdif f (s−1)
(kcal/mol) (K) 10 K 50 K H 0.7 350 5.100×10+04 3.62×10+05 C 1.6 800 4.855×10−05 1.07×10+04 O 1.6 800 4.207×10−05 9.24×10+03 CH 1.3 654 3.377×10−03 2.22×10+04 CH2 1.9 956 4.581×10−07 4.23×10+03 CO 2.4 1207 1.920×10−10 7.44×10+02 OH 2.5 1258 5.557×10−11 7.20×10+02 HCO 3.0 1509 2.456×10−14 1.34×10+02 NH2 1.7 855 8.302×10−06 6.85×10+03 CH3 2.3 1157 1.161×10−09 1.34×10+03 CH3O 3.4 1710 6.030×10−17 4.11×10+01 CH2OH 4.3 2163 8.523×10−23 3.05×10+00 COOH 4.0 2012 6.324×10−21 6.04×10+00 HCOOH 5.1 2565 4.335×10−28 2.44×10−01 H2CO 3.5 1761 1.375×10−17 3.13×10+01 NH2CO 3.9 1962 2.857×10−20 8.16×10+00 CH3CO 4.7 2364 1.804×10−25 8.10×10−01 CH3OCO 6.2 3119 2.590×10−35 8.57×10−03 HOCH2CO 6.9 3471 7.032×10−40 1.09×10−03 NH2CHO 4.7 2364 1.764×10−25 7.91×10−01 CH3CHO 5.4 2716 4.920×10−30 1.04×10−01 CH3OCHO 6.5 3270 2.838×10−37 3.52×10−03 HOCH2CHO 7.4 3722 3.838×10−43 2.50×10−04
Note: Quantum tunneling dominates over diffusion for H at 10 K, and so the H tunneling rate is given at this temperature.
reinforced by recent observational studies of interstellar ices, which have abundance ratios of H2O:CO2:H2CO:CO:CH3OH:NH3 of 100:18:12:10:8:7 [83].
At 50 K, however, the diffusion rates of the other radicals increase significantly, and so more complex species could form in this type of environment if the diffusion rates are comparable to the arrival rate of H from the gas phase. The hydrogen accretion rate from the gas phase can be calculated by the following equation:
dnH,grain/dt=πr2(2kT /m)1/2nH,gasmζng (8.6)
The gas phase hydrogen density,nH,gas, can be approximated as 2×10−4 nT,gas, andnT,gas is on the order of∼104 cm−3. A sticking coefficient,ζ, of unity, an average grain radius, r, of 1×10−7 m, and a grain density, ng, of 10−12 nT,gas can also be assumed. The flux of H from the gas phase is therefore on the order of 2×10−7 s−1 at both 10 and 50 K. The 50 K diffusion rates in Table 8.6 of the more complex radicals are indeed higher than this arrival rate, and so complex chemistry involving these species is certainly possible. Indeed, recent observations of UV-processed interstellar ices with the Spitzer Space Telescope reveal that HCOOH is enhanced in such regions [84].
The photolysis products presented in Table 8.1 are clearly important in the formation of compounds such as methyl formate and glycolaldehyde as well as all of the 3C compounds, and so their reactions should certainly be included in grain surface models. The 2C structural isomers methyl formate, acetic acid, and glycolaldehyde could indeed form in significant quantities from these processes. Direct comparisons can be made for these simpler species formed from radical-radical combinations using the information derived above and the observed interstellar ratios for the starting material. An analysis of the relative reaction rates of the HCO + radical combination reactions results in the abundance ratios shown in Table 8.7. The branching ratios discussed above were combined with a CH3 production pathway branching ratio of 0.1 for the purposes of this calculation. The assumption was made here that the available H on the grain surface would be determined by its production from photolysis processes. H will be in steady flux between the grain surface and gas phase, and so this number is an underestimate of the total amount of H on the grain surface. This estimation, however, provides an upper limit for the amount of more complex species that could form in such environments.
This comparison shows that the calculated reaction rate coefficients may in fact be
Table 8.7: Observed and calculated abundance ratios for the products of HCO+radical combination reactions relative to formaldehyde at 50 K. The observed column densities are those determined for Sgr B2(N-LMH).
NT/NT,f ormaldehyde
Formula Species Observed Predicted at 50 K
H2CO formaldehyde 1 1
NH2CHO formamide 6 1.6×10−03 CH3CHO acetaldehyde 21 8.8×10−06 HCOOH formic acid 0.3 2.3×10−03 CH3OCHO methyl formate 4 8.9×10−06 HOCH2CHO glycolaldehyde 0.1 3.3×10−07
underestimated if the abundance ratios in hot cores are truly linked to grain surface mechanisms. It is likely, however, that the simpler, more reactive species such as formic acid and glycolaldehyde may undergo more complex reactions either on the grain or in the gas phase in the hot core, and so these observed abundances may not truly reflect grain surface composition. Regardless, abstraction pathways are clearly competitive on grain surfaces in warm regions, and these types of reactions should be integrated into current astrochemical models.
It is also likely that the 3C species discussed in this thesis could be formed on grains if abstraction reactions can compete with radical-radical combination reactions and single- atom addition reactions. It is clear from the rates presented in Table 8.4 that hydrogen will dominate both formation and abstraction reactions at both temperatures, so formaldehyde will likely be the dominant product of such channels at 10 K. As is demonstrated by the analysis presented in Table 8.7, however, the other radical reaction channels with HCO are also possible at 50 K, and so other complex aldehydes will likely be present in warmer regions.
Theab initiostudies of radical-aldehyde interactions indicate that hydrogen abstraction
reactions have much lower barriers than do addition reactions involving the carbonyl group [77]. The hydrogen abstraction routes alone are therefore enough to compete with the single-atom addition reactions considered in other models. Once the aldehyde radicals are formed, these species could recombine with any of the mobile radicals. The products of recombination with hydrogen will be the primary products of these reactions, but the more complex pathways involving heavier radicals are also possible at 50 K. Species such as dihydroxyacetone, dimethyl carbonate, and methyl glycolate may well form from such mechanisms, and an analysis similar to that conducted for the simpler species in Table 8.7 can be used to investigate the predicted relative ratios of these isomers. Such an analysis reveals that the relative ratios of these species should be roughly 1:8300:8000, respectively. These results follow the trend reflected by the observational results, and once again demonstrate the need for aldehyde proton abstraction reactions to be incorporated into grain surface models.
It is clear from these preliminary analyses that grain surface chemistry has the potential to achieve considerable complexity. H addition reactions dominate the grain surface chemistry at low temperature, forming simple species such as water, methanol, and formaldehyde. Photolysis of simple grain mantle constituents leads to the production of surface radicals that can efficiently compete with H addition reactions at warmer temperatures, and so periodic thermal processing of grain mantles will lead to the buildup of more complex species such as formic acid, methyl formate, formamide, acetaldehyde, and glycolaldehyde. Aldehyde proton abstraction reactions can efficiently compete with single- atom addition reactions at both low and high temperatures, and so the mobile radicals can then react with the resultant aldehyde radicals to form more complex species such as those investigated in this thesis. Simpler species will be favored at low temperature, but
these radicals may also be stored in the grain mantle at low temperature and undergo more complex reactions upon grain mantle heating in hot core regions.