Issue |
A&A
Volume 699, July 2025
|
|
---|---|---|
Article Number | A126 | |
Number of page(s) | 10 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/202452470 | |
Published online | 02 July 2025 |
Impact of cosmic-ray propagation on the chemistry and ionisation fraction of dark clouds
1
Departamento de Astronomía, Facultad de Ciencias Físicas y Matemáticas, Universidad de Concepción, Av. Esteban Iturra s/n Barrio Universitario,
Casilla 160,
Concepción,
Chile
2
Como Lake Center for Astrophysics, DiSAT, Università degli Studi dell’Insubria,
via Valleggio 11,
22100
Como,
Italy
3
INAF, Osservatorio Astronomico di Bologna,
Via Gobetti 93/3,
40129
Bologna,
Italy
4
Chemistry Department, Sapienza University of Rome,
P.le A. Moro,
00185
Rome,
Italy
5
INAF – Osservatorio Astrofisico di Arcetri,
Largo E. Fermi 5,
50125
Firenze,
Italy
6
Max-Planck-Institut für Extraterrestrische Physik,
Giessenbachstr. 1,
85749
Garching bei München,
Germany
★ Corresponding author: gonzajaque2016@udec.cl
Received:
3
October
2024
Accepted:
30
April
2025
Aims. A proper modelling of the cosmic-ray ionisation rate within gas clouds is crucial to describe their chemical evolution accurately. However, this modelling is computationally demanding because it requires the propagation of cosmic rays throughout the cloud over time. We present a more efficient approach that simultaneously guarantees a reliable estimate of the cosmic-ray impact on the chemistry of prestellar cores.
Methods. We introduce a numerical framework that mimics the cosmic-ray propagation within gas clouds and applies it to magnetohydrodynamic simulations performed with the code GIZMO. It simulates the cosmic-ray attenuation by computing the effective column density of H2 that is traversed, which is estimated using the same kernel weighting approach as employed in the simulation. The obtained cosmic-ray ionisation rate is then used in post-processing to study the chemical evolution of the clouds.
Results. We found that cosmic-ray propagation affects deuterated and non-deuterated species significantly and that it depends on the assumed cosmic-ray spectrum. We explored correlations between the electron abundance, the cosmic-ray ionisation rate, and the abundance of the most relevant ions (HCO+, N2H+, DCO+, N2D+, and o-H2D+), with the purpose of finding simple expressions that link them. We provide an analytical formula to estimate the ionisation fraction, X(e−), from observable tracers and applied it to existing observations of high-mass clumps. We obtained values of about 10−8, which is in line with previous works and with expectations for dense clouds. We also provide a linear fit to calculate the cosmic-ray ionisation rate from the local H2 density, which is to be employed in three-dimensional simulations that do not include cosmic-ray propagation.
Key words: ISM: abundances / ISM: clouds / cosmic rays / ISM: molecules
© The Authors 2025
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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