Chemistry as a probe of the structures and evolution of massive star-forming regions

We present detailed thermal and gas-phase chemical models for the envelope of the massive star-forming region AFGL 2591. By considering both time- and space-dependent chemistry, these models are used to study both the physical structure proposed by van der Tak et al. (1999, 2000), as well as the chemical evolution of this region. The model predictions are compared with observed abundances and column densities for 29 species. The observational data cover a wide range of physical conditions within the source, but significantly probe the inner regions where interesting high-temperature chemistry may be occurring. Taking appropriate care when comparing models with both emission and absorption measurements, we find that the majority of the chemical structure can be well-explained. In particular, we find that the nitrogen and hydrocarbon chemistry can be significantly affected by temperature, with the possibility of high-temperature pathways to HCN. While we cannot determine the sulphur reservoir, the observations can be explained by models with the majority of the sulphur in CS in the cold gas, SO2 in the warm gas, and atomic sulphur in the warmest gas. Because the model overpredicts CO2 by a factor of 40, various high-temperature destruction mechanisms are explored, including impulsive heating events. The observed abundances of ions such as HCO+ and N2H+ and the cold gas-phase production of HCN constrain the cosmic-ray ionization rate to similar to5.6 x 10(17) s(-1), to within a factor of three. Finally, we find that the model and observations can simultaneously agree at a reasonable level and often to within a factor of three for 7 x 10(3) less than or equal to t(yrs) less than or equal to 5 x 10(4), with a strong preference for t similar to 3 x 10(4) yrs since the collapse and formation of the central luminosity source.