Dependence of Chemical Abundance on the Cosmic Ray Ionization Rate in IC 348

Abstract

Ions (e.g., H${}_3^{+}$, H${}_2$O${}^{+}$) have been used extensively to quantify the cosmic-ray ionization rate (CRIR) in diffuse sightlines. However, measurements of CRIR in low-to-intermediate density gas environments are rare, especially when background stars are absent. In this work, we combine molecular line observations of CO, OH, CH, and HCO${}^{+}$ in the star-forming cloud IC~348, and chemical models to constrain the value of CRIR and study the response of the chemical abundances distribution. The cloud boundary is found to have an $A_{\mathrm{V}}$ of approximately 4 mag. From the interior to the exterior of the cloud, the observed ${}^{13}$CO line intensities drop by an order of magnitude. The calculated average abundance of ${}^{12}$CO (assuming ${}^{12}$C/${}^{13}$C = 65) is (1.2$\pm$0.9) $\times$10${}^{-4}$, which increases by a factor of 6 from the interior to the outside regions. The average abundance of CH (3.3$\pm$0.7 $\times$ 10${}^{-8}$) is in good agreement with previous findings in diffuse and translucent clouds ($A_{\mathrm{V}}$ $<$ 5 mag). However, we did not find a decline in CH abundance in regions of high extinction ($A_{\mathrm{V}}\simeq$8 mag) as previously reported in Taurus. By comparing the observed molecular abundances and chemical models, we find a decreasing trend of CRIR as $A_{\mathrm{V}}$ increases. The inferred CRIR of $\hat{I}{\P }_{cr}$ = (4.7$\pm$1.5) $\times$ 10${}^{-16}$ s${}^{-1}$ at low $A_{\mathrm{V}}$ is consistent with H${}_3^{+}$ measurements toward two nearby massive stars.

21 pages, 11 figures. Submitted to ApJ