Issue |
A&A
Volume 658, February 2022
|
|
---|---|---|
Article Number | A151 | |
Number of page(s) | 11 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/202142411 | |
Published online | 11 February 2022 |
The impact of cosmic-ray attenuation on the carbon cycle emission in molecular clouds
1
I. Physikalisches Institut, Universität zu Köln,
Zülpicher Straße 77,
50937
Köln,
Germany
e-mail: gaches@ph1.uni-koeln.de
2
Center of Planetary Systems Habitability, The University of Texas at Austin,
USA
3
Department of Astronomy, University of Maryland,
College Park,
MD
20742,
USA
Received:
11
October
2021
Accepted:
11
January
2022
Context. Observations of the emission of the carbon cycle species (C, C+, CO) are commonly used to diagnose gas properties in the interstellar medium, but they are significantly sensitive to the cosmic-ray ionization rate. The carbon-cycle chemistry is known to be quite sensitive to the cosmic-ray ionization rate, ζ, controlled by the flux of low-energy cosmic rays which get attenuated through molecular clouds. However, astrochemical models commonly assume a constant cosmic-ray ionization rate in the clouds.
Aims. We investigate the effect of cosmic-ray attenuation on the emission of carbon cycle species from molecular clouds, in particular the [CII] 158 μm, [CI] 609 μm, and CO (J = 1–0) 115.27 GHz lines.
Methods. We used a post-processed chemical model of diffuse and dense simulated molecular clouds and quantified the variation in both column densities and velocity-integrated line emission of the carbon cycle with different cosmic-ray ionization rate models.
Results. We find that the abundances and column densities of carbon cycle species are significantly impacted by the chosen cosmic-ray ionization rate model: no single constant ionization rate can reproduce the abundances modeled with an attenuated cosmic-ray model. Further, we show that constant ionization rate models fail to simultaneously reproduce the integrated emission of the lines we consider, and their deviations from a physically derived cosmic-ray attenuation model is too complex to be simply corrected. We demonstrate that the two clouds we modeled exhibit a similar average AV,eff – nH relationship, resulting in an average relation between the cosmic-ray ionization rate and density ζ(nH).
Conclusions. We conclude by providing a number of implementation recommendations for cosmic rays in astrochemical models, but emphasize the necessity for column-dependent cosmic-ray ionization rate prescriptions.
Key words: astrochemistry / ISM: abundances / ISM: clouds / cosmic rays / ISM: molecules
© ESO 2022
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