Chemical differentiation in regions of massive star formation

We have reexamined the origin of the apparent differentiation between nitro gen-bearing molecules and complex oxygen-bearing molecules that is observed in hot molecular cores associated with massive protostars. Observations sh ow that methanol is an ubiquitous and abundant component of protostellar ic es. Recent observations suggest that ammonia may constitute an appreciable fraction of the ices toward some sources. In contrast to previous theories that suggested that N/O differentiation was caused by an anticorrelation be tween methanol and ammonia in the precursor grain mantles, we show that the presence of ammonia in mantles and the core temperature are key quantities in determining N/O differentiation. Calculations are presented which show that when large amounts of ammonia are evaporated alkyl cation transfer rea ctions are suppressed and the abundances of complex O-bearing organic molec ules greatly reduced. Cooler cores (100 K) eventually evolve to an oxygen-r ich chemical state similar to that attained when no ammonia was injected, b ut on a timescale that is an order of magnitude longer (similar to 10(5) yr ). Hotter cores (300 K) never evolve an O-rich chemistry unless ammonia is almost absent from the mantles. In this latter case, a complex O-rich chemi stry develops on a timescale of similar to 10(4) yr, as in previous models, but disappears in about 2 X 10(5) yr, after which time the core is rich in NH3, HCN, and other N-bearing molecules. There are thus two ways in which N-rich cores can occur. We briefly discuss the implications for the determi nation of hot-core ages and for explaining N/O differentiation in several w ell-studied sources.