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

Abstract

Ions (e.g., ${{\rm{H}}}_{3}^{+}$ , H _2 O ^+ ) have been used extensively to quantify the cosmic-ray ionization rate (CRIR) in diffuse sightlines. However, measurements of the 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 the CRIR and study the response of the distribution of chemical abundances. The cloud boundary is found to have an A _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 ± 0.9) × 10 ^−4 , which decreases by a factor of 6 from the interior to the outside regions. The average abundance of CH (3.3 ± 0.7 × 10 ^−8 ) is in good agreement with previous findings in diffuse and translucent clouds ( A _V < 5 mag). However, we did not find a decline in CH abundance in regions of high extinction ( A _V ≃ 8 mag) as previously reported in Taurus. By comparing the observed molecular abundances and chemical models, we find a decreasing trend of the CRIR as A _V increases. The inferred CRIR of ζ _cr = (4.7 ± 1.5) × 10 ^−16 s ^−1 at low A _V is consistent with ${{\rm{H}}}_{3}^{+}$ measurements toward two nearby massive stars.

Keywords

• Interstellar medium
• Interstellar molecules
• Chemical abundances
• Molecular clouds