Letter to the Editor

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

¹ INAF – Istituto di Radioastronomia & Italian ALMA Regional Centre, Via P. Gobetti 101, 40129 Bologna, Italy
e-mail: agianne@ira.inaf.it
² Departamento de Astronomía, Universidad de Concepción, Barrio Universitario, Concepción, Chile
³ Centre for Astrochemical Studies, Max-Planck-Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ INAF-Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy
⁵ Max-Planck-Institut für Radioastronomie, auf dem Hügel 69, 53121 Bonn, Germany
⁶ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁷ Institute for Astrophysical Research, Boston University, 725 Commonwealth Ave, Boston, MA 02215, USA

Received: 7 November 2018
Accepted: 18 December 2018

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 H₃⁺ and its deuterated isotopologues become more abundant, enhancing the deuterium fraction 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 372 GHz, and N₂D⁺(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-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 efficient tracer of the evolutionary stages of the star-formation process.