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

¹ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
² Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
³ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Firenze, Italy
⁴ Chemistry Department, Sapienza University of Rome, P.le A. Moro, 00185 Rome, Italy
⁵ Departamento de Astronomía, Facultad Ciencias Físicas y Matemáticas, Universidad de Concepción, Av. Esteban Iturra s/n Barrio Universitario, Casilla 160, Concepción, Chile
⁶ INAF, Istituto di Radioastronomia – Italian node of the ALMA Regional Centre (It-ARC), Via Gobetti 101, 40129 Bologna, Italy
⁷ Institute for Advanced Study, Kyushu University, Kyushu, Japan
⁸ Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan
⁹ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁰ Max-Planck-Institut für Radioastronomie, Auf dem Hügel, 69, 53121 Bonn, Germany

^★ Corresponding author.

Received: 28 November 2024
Accepted: 22 August 2025

Abstract

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.