2014A&A...572A..63I

The object W 33 is a giant molecular cloud that contains star forming regions at various evolutionary stages from quiescent clumps to developed HII regions. Since its star forming regions are located at the same distance and the primary material of the birth clouds is probably similar, we conducted a comparative chemical study to trace the chemical footprint of the different phases of evolution. We observed six clumps in W 33 with the Atacama Pathfinder Experiment (APEX) telescope at 280GHz and the Submillimeter Array (SMA) at 230GHz. We detected 27 transitions of 10 different molecules in the APEX data and 52 transitions of 16 different molecules in the SMA data. The chemistry on scales larger than ∼0.2pc, which are traced by the APEX data, becomes more complex and diverse the more evolved the star forming region is. On smaller scales traced by the SMA data, the chemical complexity and diversity increase up to the hot core stage. In the HII region phase, the SMA spectra resemble the spectra of the protostellar phase. Either these more complex molecules are destroyed or their emission is not compact enough to be detected with the SMA. Synthetic spectra modelling of the H₂CO transitions, as detected with the APEX telescope, shows that both a warm and a cold component are needed to obtain a good fit to the emission for all sources except for W 33 Main 1. The temperatures and column densities of the two components increase during the evolution of the star forming regions. The integrated intensity ratios N₂H⁺(3-2)/CS(6-5) and N₂H⁺(3-2)/H₂CO(4_2,2-3_2,1) show clear trends as a function of evolutionary stage, luminosity, luminosity-to-mass ratio, and H₂ peak column density of the clumps and might be usable as chemical clocks.