Simulations of Hot-Core Chemistry

The past decade has seen great advances in the simulation of hot-core chemistry. Significant efforts have been made to understand the formation of complex organic molecules in starforming regions, in light of the apparent deficiency of gas-phase processes. As a result, a greater emphasis has been placed on the detailed treatment of grain-surface and bulk-ice processes and their interactions with the gas phase. Chemical kinetics models based on rate equations can be used to examine the complex chemistry of hot-core regions, and great advances have been made in the implementation of Monte Carlo simulations, although, in both cases, the treatment of detailed physical structure within the dust-grain ice mantles remains a challenge. Sophisticated models are now routinely used to combine gasphase, grain-surface, and bulk-ice chemical treatments, while their integration with more realistic physical models of specific sources has become important for the simulation of molecular emission spectra. Additionally, chemical networks used for hot-core models now incorporate chemistry of direct biological significance, including formation mechanisms for amino acids and sugars. The results of these modeling studies can be directly compared to the complexity observed with the newest generation of observational instruments, which provide quantitative information that can be benchmarked against the models. Despite these successes, there remains a large gap between current modeling capabilities and the objective of a full, comprehensive model of star-formation environments. Chemical reaction networks are far from complete, and many of the parameters therein are educated guesses at best, while detailed treatments of ice structure are yet in their infancy. Likewise, physical models that properly account for the hydrodynamical processes occurring in hot cores have not yet been coupled with the more comprehensive chemical networks necessary to examine the chemistry of star-forming regions; indeed, the dynamics of high-mass star formation are currently a matter of considerable debate. Nonetheless, the results from the current chemical models of hot cores are valuable tools for comparison with observations; the simulations agree well with observations for the most abundant molecular species, and advances in the translation of chemical model results into directly observable quantities will allow more specific predictions to be made for individual sources. In the coming years, models of hot-core chemistry will need to advance and expand in several different directions, both to address current challenges and to incorporate new information from high-quality astronomical observations and chemical experiments. However, one might also expect that these advances will place yet greater technical demands on the computational models, some of which already operate close to the limits of feasible run times. The breadth of chemical and physical processes considered, and the resultant constraints placed upon the models, will require that astrochemists not be too dogmatic in the demand that every part of a model be stateof- the-art; even with the best computers in the world, such models of astrophysical sources will never be entirely comprehensive. The choice of whether detailed chemistry, detailed ice structure, detailed dynamics, and/or detailed radiative transfer are used must depend on the choice of problem and the abilities of individual scientists to best exploit their own capabilities. However, in spite of the inevitable incompleteness of astrochemical models, the past few years have demonstrated that the models can reliably reproduce many facets of the observational data, that they can explain the microscopic processes occurring on astronomical scales, and that they can be used to guide future observational strategies for hot-core sources.