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The impact of attenuation on cosmic-ray chemistry: I. Abundances and chemical calibrators in molecular clouds

Arghyadeb Roy, Brandt A. L. Gaches, Jonathan C. Tan

Abstract

The chemistry of shielded molecular gas is primarily driven by energetic, charged particles dubbed cosmic rays (CRs), in particular those with energies under 1 GeV. CRs ionize molecular hydrogen and helium, the latter of which contributes greatly to the destruction of molecules. CR ionization initiates a wide range of gas-phase chemistry, including pathways important for the so-called "carbon cycle", C$^+$/C/CO. Therefore, the CR ionization rate, $ζ$, is fundamental in theoretical and observational astrochemistry. Although observational methods show a wide range of ionization rates -- varying with the environment, especially decreasing into dense clouds -- astrochemical models often assume a constant rate. To address this limitation, we employ a post-processed gas-phase chemical model of a simulated dense molecular cloud that incorporates CR energy losses within the cloud. This approach allows us to investigate changes in abundance profiles of important chemical tracers and gas temperature. Furthermore, we analyze analytical calibrators for estimating $ζ$ in dense molecular gas that are robust when tested against a full chemical network. Additionally, we provide improved estimations of the electron fraction in dense gas for better consistency with observational data and theoretical calibrations for UV-shielded regions.

The impact of attenuation on cosmic-ray chemistry: I. Abundances and chemical calibrators in molecular clouds

Abstract

The chemistry of shielded molecular gas is primarily driven by energetic, charged particles dubbed cosmic rays (CRs), in particular those with energies under 1 GeV. CRs ionize molecular hydrogen and helium, the latter of which contributes greatly to the destruction of molecules. CR ionization initiates a wide range of gas-phase chemistry, including pathways important for the so-called "carbon cycle", C/C/CO. Therefore, the CR ionization rate, , is fundamental in theoretical and observational astrochemistry. Although observational methods show a wide range of ionization rates -- varying with the environment, especially decreasing into dense clouds -- astrochemical models often assume a constant rate. To address this limitation, we employ a post-processed gas-phase chemical model of a simulated dense molecular cloud that incorporates CR energy losses within the cloud. This approach allows us to investigate changes in abundance profiles of important chemical tracers and gas temperature. Furthermore, we analyze analytical calibrators for estimating in dense molecular gas that are robust when tested against a full chemical network. Additionally, we provide improved estimations of the electron fraction in dense gas for better consistency with observational data and theoretical calibrations for UV-shielded regions.
Paper Structure (22 sections, 31 equations, 21 figures, 2 tables)

This paper contains 22 sections, 31 equations, 21 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Methodology
  3. Results
  4. Physical properties
  5. Chemical abundances
  6. Discussion
  7. Benchmarking H3+ analytic chemistry
  8. CRIR calibrators using carbon-cycle species
  9. Analytical estimation of electron fraction
  10. Conclusions and outlook
  11. Abundance profiles and relative errors
  12. Derivation of the carbon-cycle calibrators
  13. Steady-state derivation of the HCO$^+$/CO abundance ratio
  14. ($i$) For $\mathrm{H_2^+}$:
  15. ($ii$) For $\mathrm{H_3^+}$:
  16. ...and 7 more sections

Figures (21)

  • Figure 1: Mass weighted local CRIR---density 2D-PDF for the attenuated High $(\mathcal{H})$ and Low $(\mathcal{L})$ models applied in the "dense" cloud physical model (see text). The constant CRIR values are denoted as dashed lines.
  • Figure 2: Mass-weighted density---temperature phase plot in semi-logarithmic scale (logarithmic in density). Density increases from left to right. The CR model is annotated in each panel.
  • Figure 3: CO abundance profile, $\chi(\text{CO})$ versus effective ionization rate, $\zeta/n_\text{H}$. Density increases from right to left. The CR model is annotated in each panel. The red lines denote the binned averaged abundance profiles in log-log space. The dashed-dotted lines show the $\zeta(N)_\mathcal{H}$ model, and dashed lines show the $\zeta(N)_\mathcal{L}$ model abundance profile, for comparison.
  • Figure 4: As in Fig. \ref{['fig:CO_abund']}, but for HCO+ abundance profile. Density increases from right to left.
  • Figure 5: As in Fig. \ref{['fig:CO_abund']}, but for N2H+ abundance profile. Density increases from right to left.
  • ...and 16 more figures