What can ISO tell us about gas–grain chemistry?

Abstract

Recent results of searches for IR absorption lines of gas-phase molecules in the ISO spectra of embedded massive young stars are discussed. Abundant, highly excited gas-phase H₂O is detected in the ‘hot cores’ surrounding the young stellar objects, but not in the colder regions. The abundance of gas-phase CO₂ is low in all sources, in spite of the large observed abundance of solid CO₂. Gas-phase CH₄, C₂H₂ and HCN have been observed as well. The latter two species have high excitation temperatures, of the order of 1000 K, in some sources. The large abundance of CH₄ suggests that the molecule is formed mainly through grain-surface reactions. CO₂ may be produced by grain-surface chemistry as well, athough photochemical production in ices can also play a role. Comparison of gas-phase with solid-state column densities for the same lines of sight allows accurate determination of gas:solid state ratios, which can be analyzed as functions of the temperature of the region. The data indicate that significant evaporation of ices likely occurs in the hotter regions, but that part of the observed gas-phase H₂O may also be produced by high-temperature gas-phase reactions. The results are discussed in the context of the physical evolution of the regions.