A low cosmic-ray ionisation rate in the pre-stellar core Ophiuchus/H-MM1. Mapping of the molecular ions ortho-H₂D⁺, N₂H⁺, and DCO⁺

Abstract

Aims: We test the use of three common molecular ions, ortho-H₂D⁺ (oH₂D⁺), N₂H⁺, and DCO⁺, as probes of the internal structure and kinematics of a dense, starless molecular cloud core.
Methods: The pre-stellar core H-MM1 in Ophiuchus was mapped in the oH₂D⁺(1₁₀ − N₂H⁺(4 − 3), and DCO⁺ (5 − 4) lines with the Large APEX sub-Millimeter Array (LAsMA) multi-beam receiver of the Atacama Pathfinder EXperiment (APEX) telescope. We also ran a series of chemistry models to predict the abundance distributions of the observed molecules, and to estimate the effect of the cosmic-ray ionisation rate on their abundances.
Results: The three line maps show different distributions. The oH₂D⁺ map is extended and outlines the general structure of the core, N₂H⁺ mainly shows the density maxima, and the DCO⁺ emission peaks are shifted towards one edge of the core where a region of enhanced desorption had previously been found. According to the chemical simulation, the fractional oH₂D⁺ abundance remains relatively high in the centre of the core, and its column density correlates strongly with the cosmic-ray ionisation rate, ζ_H2. Simulated line maps constrain the cosmic-ray ionisation rate to be low, between 5 × 10⁻¹⁸ s⁻¹ and 1 × 10⁻¹⁷ s⁻¹ in the H-MM1 core. This estimate agrees with the gas temperature measured in the core.
Conclusions: The present observations show that very dense, cold gas in molecular clouds can be traced by mapping the ground-state line of oH₂D⁺ and high-J transitions of DCO⁺ and N₂H⁺, despite the severe depletion of the latter two molecules. Modelling line emission of oH₂D⁺ provides a straightforward method of determining the cosmic-ray ionisation rate in dense clouds, where the primary ion, H₃⁺, is not observable.

This publication is based on data acquired with the Atacama Pathfinder Experiment (APEX). APEX is operated by the European Southern Observatory (ESO) on behalf of the Max-Planck-Institut für Radioastronomie (MPIfR).