A timeline for massive star-forming regions via combined observation of o-H${}_2$D${}^{+}$ and N${}_2$D${}^{+}$

Abstract

Context: In cold and dense gas prior to the formation of young stellar objects, heavy molecular species (including CO) are accreted onto dust grains. Under these conditions H${}_3^{+}$ and its deuterated isotopologues become more abundant, enhancing the deuterium fraction of molecules such as N${}_2$H${}^{+}$ 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-H${}_2$D${}^{+}$ and N${}_2$D${}^{+}$ vary with evolution in high-mass clumps. Methods: We observed with APEX the ground-state transitions of o-H${}_2$D${}^{+}$ near 372 GHz, and N${}_2$D${}^{+}$(3-2) near 231 GHz for three massive clumps in different 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 effects. 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-H${}_2$D${}^{+}$ and that of N${}_2$D${}^{+}$, 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 10${}^5$ 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-H${}_2$D${}^{+}$ and N${}_2$D${}^{+}$ can act as an efficient tracer of the evolutionary stages of the star-formation process.

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• The ALMA Survey of 70 $\mu \mathrm{m}$ Dark High-mass Clumps in Early Stages (ASHES). VII: Chemistry of Embedded Dense Cores
• The 3D structure of CO depletion in high-mass prestellar regions
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• APEX observations of ortho-H${}_2$D${}^{+}$ towards dense cores in the Orion B9 filament
• Deuterium Fractionation across the Infrared Dark Cloud G034.77-00.55 interacting with the Supernova Remnant W44
• Benchmarking the IRDC G351.77-0.53: Gaia DR3 distance, mass distribution, and star formation content
• Characterization of dense Planck clumps observed with Herschel and SCUBA-2
• Molecular deuterations in massive Starless Clump Candidates
• Synthetic observations using POLARIS: an application to simulations of massive prestellar cores
• Chemical analysis of prestellar cores in Ophiuchus yields short timescales and rapid collapse