A timeline for massive star-forming regions via combined observation of o-H2D+ and N2D+

Context. In cold and dense gas prior to the formation of young stellar objects, heavy molecular species (including CO) are accreted ontodustgrains.UndertheseconditionsH+3 anditsdeuteratedisotopologuesbecomemoreabundant,enhancingthedeuteriumfraction of molecules such as N2H+ that are formed via ion-neutral reactions. Because this process is extremely temperature sensitive, the abundance of these species is likely linked to the evolutionary stage of the source. Aims. We investigate how the abundances of o-H2D+ and N2D+ vary with evolution in high-mass clumps. Methods. We observed with APEX the ground-state transitions of o-H2D+ near 372GHz, and N2D+(3–2) near 231GHz for three massive clumps in di↵erent evolutionary stages. The sources were selected within the G351.77–0.51 complex to minimise the variation of initial chemical conditions, and to remove distance e↵ects. We modelled their dust continuum emission to estimate their physical properties, and also modelled their spectra under the assumption of local thermodynamic equilibrium to calculate beam-averaged abundances. Results. We find an anticorrelation between the abundance of o-H2D+ and that of N2D+, with the former decreasing and the latter increasing with evolution. With the new observations we are also able to provide a qualitative upper limit to the age of the youngest clump of about 105 yr, comparable to its current free-fall time. Conclusions. We can explain the evolution of the two tracers with simple considerations on the chemical formation paths, depletion of heavy elements, and evaporation from the grains. We therefore propose that the joint observation and the relative abundance of o-H2D+ and N2D+ can act as an ecient tracer of the evolutionary stages of the star-formation process.