Establishing the evolutionary timescales of the massive star formation process through chemistry

Context. Understanding the details of the formation process of massive (i.e. M greater than or similar to 8-10 M-circle dot) stars is a long-standing problem in astrophysics. They form and evolve very quickly, and almost their entire formation process takes place deeply embedded in their parental clumps. Together with the fact that these objects are rare and at a relatively large distance, this makes observing them very challenging. Aims. We present a method for deriving accurate timescales of the evolutionary phases of the high-mass star formation process. Methods. We modelled a representative number of massive clumps of the ATLASGAL-TOP100 sample that cover all the evolutionary stages. The models describe an isothermal collapse and the subsequent warm-up phase, for which we followed the chemical evolution. The timescale of each phase was derived by comparing the results of the models with the properties of the sources of the ATLASGAL-TOP100 sample, taking into account the mass and luminosity of the clumps, and the column densities of methyl acetylene (CH3CCH), acetonitrile (CH3CN), formaldehyde (H2CO), and methanol (CH3OH). Results. We find that the molecular tracers we chose are affected by the thermal evolution of the clumps, showing steep ice evaporation gradients from 10(3) to 10(5) AU during the warm-up phase. We succeed in reproducing the observed column densities of CH3CCH and CH3CN, but H2CO and CH3OH agree less with the observed values. The total (massive) star formation time is found to be similar to 5.2 x 10(5) yr, which is defined by the timescales of the individual evolutionary phases of the ATLASGAL-TOP100 sample: similar to 5 x 10(4) yr for 70-mu m weak, similar to 1.2 x 10(5) yr for mid-IR weak, similar to 2.4 x 10(5) yr for mid-IR bright, and similar to 1.1 x 10(5) yr for HII-region phases. Conclusions. With an appropriate selection of molecular tracers that can act as chemical clocks, our model allows obtaining robust estimates of the duration of the individual phases of the high-mass star formation process. It also has the advantage of being capable of including additional tracers aimed at increasing the accuracy of the estimated timescales.