Volume 658, February 2022
|Number of page(s)||11|
|Section||Interstellar and circumstellar matter|
|Published online||11 February 2022|
The impact of cosmic-ray attenuation on the carbon cycle emission in molecular clouds
I. Physikalisches Institut, Universität zu Köln,
Zülpicher Straße 77,
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
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
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.