Chemical evolution in the early phases of massive star formation. I
  Understanding the chemical evolution of young (high-mass) star-forming
regions is a central topic in star formation research. Chemistry is employed as
a unique tool 1) to investigate the underlying physical processes and 2) to
characterize the evolution of the chemical composition. We observed a sample of
59 high-mass star-forming regions at different evolutionary stages varying from
the early starless phase of infrared dark clouds to high-mass protostellar
objects to hot molecular cores and, finally, ultra-compact HII regions at 1mm
and 3mm with the IRAM 30m telescope. We determined their large-scale chemical
abundances and found that the chemical composition evolves along with the
evolutionary stages. On average, the molecular abundances increase with time.
We modeled the chemical evolution, using a 1D physical model where density and
temperature vary from stage to stage coupled with an advanced gas-grain
chemical model and derived the best-fit chi^2 values of all relevant
parameters. A satisfying overall agreement between observed and modeled column
densities for most of the molecules was obtained. With the best-fit model we
also derived a chemical age for each stage, which gives the timescales for the
transformation between two consecutive stages. The best-fit chemical ages are
~10,000 years for the IRDC stage, ~60,000 years for the HMPO stage, ~40,000
years for the HMC stage, and ~10,000 years for the UCHII stage. The total
chemical timescale for the entire evolutionary sequence of the high-mass star
formation process is on the order of 10^5 years, which is consistent with
theoretical estimates. Furthermore, based on the approach of a multiple-line
survey of unresolved data, we were able to constrain an intuitive and
reasonable physical and chemical model. The results of this study can be used
as chemical templates for the different evolutionary stages in high-mass star
formation.
