The Astrochemical Impact of Cosmic Rays in Protoclusters II: CI-to-H${}_2$ and CO-to-H${}_2$ Conversion Factors

Abstract

We utilize a modified astrochemistry code which includes cosmic ray attenuation in-situ to quantify the impact of different cosmic-ray models on the CO-to-H${}_2$ and CI-to-H${}_2$ conversion factors, $X_{\mathrm{CO}}$ and $X_{\mathrm{CI}}$, respectively. We consider the impact of cosmic rays accelerated by accretion shocks, and show that clouds with star formation efficiencies greater than 2\% have $X_{\mathrm{CO}}=(2.5\pm 1)\times 10^{20}$ cm${}^{-2}$(K km s${}^{-1}$)${}^{-1}$, consistent with Milky Way observations. We find that changing the cosmic ray ionization rate from external sources from the canonical $\zeta \approx 10^{-17}$ to $\zeta \approx 10^{-16}$ s${}^{-1}$, which better represents observations in diffuse gas, reduces $X_{\mathrm{CO}}$ by 0.2 dex for clusters with surface densities below 3 g cm${}^{-2}$. We show that embedded sources regulate $X_{\mathrm{CO}}$ and decrease its variance across a wide range of surface densities and star formation efficiencies. Our models reproduce the trends of a decreased $X_{\mathrm{CO}}$ in extreme cosmic ray environments. $X_{\mathrm{CI}}$ has been proposed as an alternative to $X_{\mathrm{CO}}$ due to its brightness at high redshifts. The inclusion of internal cosmic ray sources leads to 1.2 dex dispersion in $X_{\mathrm{CI}}$ ranging from $2\times 10^{20}<X_{\mathrm{CI}}<4\times 10^{21}$ cm${}^{-2}$ (K km s${}^{-1}$)${}^{-1}$. We show that $X_{\mathrm{CI}}$ is highly sensitive to the underlying cosmic ray model.

9 pages, 3 figures. ApJ Accepted

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• The impact of cosmic-ray attenuation on the carbon cycle emission in molecular clouds
• The Carbon-to-H${}_2$, CO-to-H${}_2$ Conversion Factors and Carbon Abundance on Kiloparsec Scales in Nearby Galaxies
• Transport of Protostellar Cosmic Rays in Turbulent Dense Cores
• Metallicity dependence of the CO-to-H${}_2$ and the [CI]-to-H${}_2$ conversion factors in galaxies
• Herschel 158$\mu$m [CII] Observations of "CO-dark" Gas in the Perseus Giant Molecular Cloud
• A Tentative Detection of Molecular Hydrogen (H${}_2$) Emission Lines at Cosmic Dawn