Cosmic-ray ionisation rate in low-mass cores: The role of the environment

Context. Cosmic rays drive several key processes for the chemistry and dynamical evolution of star-forming regions. Their effect is quantified mainly by means of the cosmic-ray ionisation rate ζ2. Aims. We aim to obtain a sample of ζ2 measurements in 20 low-mass, starless cores embedded in different parental clouds in order to assess the average level of ionisation in this kind of source and to investigate the role of the environment in this context. The warmest clouds in our sample are Ophiuchus and Corona Australis, where star formation activity is higher than in the Taurus cloud and the other isolated cores we targeted. Methods. We computed ζ2 using an analytical method based on the column density of ortho-H2D+, the CO abundance, and the deuteration level of HCO+. To estimate these quantities, we analysed new, high-sensitivity molecular line observations obtained with the Atacama Pathfinder EXperiment (APEX) single-dish telescope and archival continuum data from Herschel. Results. We report ζ2 estimates in 17 cores in our sample and provide upper limits on the three remaining sources. The values span almost two orders of magnitude, from 1.3 × 10−18 s−1 to 8.5 × 10−17 s−1. Conclusions. We find no significant correlation between ζ2 and the core’s column densities N(H2). On the contrary, we find a positive correlation between ζ2 and the core’s temperature, estimated via Herschel data: cores embedded in warmer environments present higher ionisation levels. The warmest clouds in our sample are Ophiuchus and Corona Australis, where star formation activity is higher than in the other clouds we targeted. The higher ionisation rates in these regions support the scenario that low-mass protostars in the vicinity of our targeted cores contribute to the re-acceleration of local cosmic rays.