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 ŗir. We aim to obtain a sample of ŗir 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. We computed ŗir using an analytical method based on the column density of ortho-̊m H₂D^+, the CO abundance, and the deuteration level of 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 We report ŗir 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 ̊m s^ to 8. 5 ̊m s^ We find no significant correlation between ŗir and the core's column densities N̊m (H₂). On the contrary, we find a positive correlation between ŗir 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.
Redaelli et al. (2025) studied this question.