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

TL;DR

This study maps molecular ions in a prestellar core to estimate the cosmic-ray ionisation rate, finding it to be low, which aligns with the core's temperature and demonstrates a method for such measurements in dense clouds.

Contribution

The paper introduces a new approach using oH2D+ line emission modeling to estimate cosmic-ray ionisation rates in dense prestellar cores.

Findings

Cosmic-ray ionisation rate in H-MM1 is between 5e-18 and 1e-17 per second.

oH2D+ abundance correlates strongly with cosmic-ray ionisation rate.

Line emission modeling provides a straightforward method for measuring ionisation rates.

Abstract

(abridged) We have mapped the prestellar core H-MM1 in Ophiuchus in rotational lines of ortho-H2D+ (oH2D+), N2H+, and DCO+ at the wavelength 0.8 mm with the Large APEX sub-Millimeter Array (LAsMA) multibeam 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. The three line maps show different distributions. The oH2D+ map is extended and outlines the general structure of the core, while 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 has been found previously. 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. Simulated line maps constrain the cosmic-ray ionisation rate per hydrogen molecule to be low, between 5e-18/s and 1e-17/s in the H-MM1 core. This estimate agrees with the gas temperature measured in the core. 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.