Deuterated forms of H${}_3^{+}$ and their importance in astrochemistry

Abstract

At the low temperatures ($\sim$10 K) and high densities ($\sim$100,000 H${}_2$ molecules per cc) of molecular cloud cores and protostellar envelopes, a large amount of molecular species (in particular those containing C and O) freeze-out onto dust grain surfaces. It is in these regions that the deuteration of H${}_3^{+}$ becomes very efficient, with a sharp abundance increase of H${}_2$D${}^{+}$ and D${}_2$H${}^{+}$. The multi-deuterated forms of H${}_3^{+}$ participate in an active chemistry: (i) their collision with neutral species produces deuterated molecules such as the commonly observed N${}_2$D${}^{+}$, DCO${}^{+}$ and multi-deuterated NH${}_3$; (ii) their dissociative electronic recombination increases the D/H atomic ratio by several orders of magnitude above the D cosmic abundance, thus allowing deuteration of molecules (e.g. CH${}_3$OH and H${}_2$O) on the surface of dust grains. Deuterated molecules are the main diagnostic tools of dense and cold interstellar clouds, where the first steps toward star and protoplanetary disk formation take place. Recent observations of deuterated molecules are reviewed and discussed in view of astrochemical models inclusive of spin-state chemistry. We present a new comparison between models based on complete scrambling (to calculate branching ratio tables for reactions between chemical species that include protons and/or deuterons) and models based on non-scrambling (proton hop) methods, showing that the latter best agree with observations of NH${}_3$ deuterated isotopologues and their different nuclear spin symmetry states.

arXiv commentaccepted for publication in the Philosophical Transactions A of the Royal Society

References in corpus (13)

• Our astrochemical heritage
• Molecular ions in L1544. II. The ionization degree
• Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+
• Observing the gas temperature drop in the high-density nucleus of L 1544
• Complete Depletion in prestellar cores
• Chemical modeling of L183 (= L134N) : an estimate of the ortho/para H2 ratio
• Multilayer formation and evaporation of deuterated ices in prestellar and protostellar cores
• HD depletion in starless cores
• Chemistry and Radiative Transfer of Water in Cold, Dense Clouds
• Spin-state chemistry of deuterated ammonia
• Modelling line emission of deuterated H_3^+ from prestellar cores
• Extended emission of D2H+ in a prestellar core
• Modelling the molecular composition and nuclear-spin chemistry of collapsing prestellar sources