The impact of cosmic-ray attenuation on the carbon cycle emission in molecular clouds

Abstract

Observations of the emission of the carbon cycle species (C, C+ CO) are commonly used to diagnose gas properties in the interstellar medium but 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, $\zeta$, 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. We investigate the effect of cosmic-ray attenuation on the emission of carbon cycle species from molecular clouds, in particular the [CII] 158 $\mu$m, [CI] 609 $\mu$m and CO (J = 1 - 0) 115.27 GHz lines. We use a post-processed chemical model of diffuse and dense simulated molecular clouds and quantify the variation in both column densities and velocity integrated line emission of the carbon cycle with different cosmic-ray ionization rate models. We find that the abundances and column densities of carbon cycle species is significantly impacted by the chosen cosmic-ray ionization rate model: no single constant ionization rate can reproduce the abundances modelled 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 model exhibit a similar average $A_{\mathrm{V},\mathrm{eff}}$ -- $n_{\mathrm{H}}$ relationship, resulting in an average relation between the cosmic-ray ionization rate and density $\zeta (n_{\mathrm{H}})$. We conclude by providing a number of implementation recommendations for CRs in astrochemical models, but emphasize the necessity for column-dependent cosmic-ray ionization rate prescriptions.

arXiv commentAccepted to A&A. Major results in Figures 4, 6, 10, 13

References in corpus (49)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Array Programming with NumPy
• Dust Grain Size Distributions and Extinction in the Milky Way, LMC, and SMC
• The CO-to-H2 Conversion Factor
• A computer program for fast non-LTE analysis of interstellar line spectra
• The UMIST database for astrochemistry 2012
• Our astrochemical heritage
• Cosmic-ray ionization of molecular clouds
• Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+
• A PDR-Code Comparison Study
• Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
• Molecular Hydrogen in the FUSE Translucent Lines of Sight: The Full Sample
• The CI lines as tracers of molecular gas, and their prospects at high redshifts
• Cosmic-ray ionisation in circumstellar discs
• Star Formation in Extreme Environments: The Effects of Cosmic Rays and Mechanical Heating
• The cosmic ray ionization rate in the Galactic disk, as determined from observations of molecular ions
• Effective destruction of CO by cosmic rays: implications for tracing H${}_2$ gas in the Universe
• SILCC-Zoom: The dynamical and chemical evolution of molecular clouds
• The Homogeneity of Interstellar Oxygen in the Galactic Disk
• ALMA observations of atomic carbon in z~4 dusty star-forming galaxies
• Cosmic-ray induced destruction of CO in star-forming galaxies
• Impact of low-energy cosmic rays on star formation
• 3D-PDR: A new three-dimensional astrochemistry code for treating Photodissociation Regions
• Isotope selective photodissociation of N2 by the interstellar radiation field and cosmic rays
• Cosmic-ray ionisation in collapsing clouds
• Chemistry in Cosmic-Ray Dominated Regions (CRDRs)
• An Alternative Accurate Tracer of Molecular Clouds: The "$X_{\mathrm{CI}}$-Factor"
• CO/H2, C/CO, OH/CO, and OH/O2 in Dense Interstellar Gas: From High Ionization to Low Metallicity
• Metallicity dependence of the H/H${}_2$ and C${}^{+}$/C/CO distributions in a resolved self-regulating interstellar medium
• Observing a column-dependent zeta in dense interstellar sources: the case of the Horsehead Nebula
• GMC Collisions as Triggers of Star Formation. II. 3D Turbulent, Magnetized Simulations
• Penetration of cosmic rays into dense molecular clouds: role of diffuse envelope
• Cosmic ray penetration in diffuse clouds
• Chemistry and radiative shielding in star forming galactic disks
• Photodissociation Region Diagnostics Across Galactic Environments
• GMC Collisions as Triggers of Star Formation. V. Observational Signatures
• The Astrochemical Impact of Cosmic Rays in Protoclusters I: Molecular Cloud Chemistry
• Physical conditions of molecular gas in the Circinus galaxy: Multi-J CO and CI 1-0 observations
• The cosmic-ray ionisation rate in the pre-stellar core L1544
• Diffusive Versus Free-Streaming Cosmic Ray Transport in Molecular Clouds
• Radiative transfer meets Bayesian statistics: where does a galaxy's [CII] emission come from?
• Synthetic [CII] emission maps of a simulated molecular cloud in formation
• Kiloparsec-Scale Simulations of Star Formation in Disk Galaxies. I. The unmagnetized and zero-feedback limit
• Observational signatures of cosmic ray interactions in molecular clouds
• Cold Clouds as Cosmic-Ray Detectors
• Transport of Protostellar Cosmic Rays in Turbulent Dense Cores
• The Astrochemical Impact of Cosmic Rays in Protoclusters II: CI-to-H${}_2$ and CO-to-H${}_2$ Conversion Factors
• Tracing H2 column density with atomic carbon (CI) and CO isotopologues
• The challenges of modelling microphysics: ambipolar diffusion, chemistry, and cosmic rays in MHD shocks

Cited by in corpus (12)

• Low energy cosmic rays
• PDFchem: A new fast method to determine ISM properties and infer environmental parameters using probability distributions
• Dependence of Chemical Abundance on the Cosmic Ray Ionization Rate in IC 348
• Abundance ratios of OH/CO and HCO+/CO as probes of the cosmic ray ionization rate in diffuse clouds
• Atacama Large Aperture Submillimeter Telescope (AtLAST) Science: Our Galaxy
• Cosmic-ray-induced H${}_2$ line emission: Astrochemical modeling and implications for JWST observations
• A new analytic approach to infer the cosmic-ray ionization rate in hot molecular cores from HCO${}^{+}$, N${}_2$H${}^{+}$, and CO observations
• Impact of cosmic-ray propagation on the chemistry and ionisation fraction of dark clouds
• An analytic formalism to describe the $N_{\mathrm{eff}}(\mathrm{H})$-$n_{\mathrm{H}}$ relationship in molecular clouds
• The impact of attenuation on cosmic-ray chemistry: I. Abundances and chemical calibrators in molecular clouds
• Tight correlation of star formation with [Ci] and CO lines across cosmic time
• Observations of ionized carbon towards the $\gamma$-ray-bright supernova remnant RX J1713.7-3946