The Astrochemical Impact of Cosmic Rays in Protoclusters. II. CI-to-H2 and CO-to-H2 Conversion Factors

We utilize a modified astrochemistry code that includes cosmic-ray (CR) attenuation in situ to quantify the impact of different CR models on the CO-to-H₂ and CI-to-H₂ conversion factors, X _CO and X _CI, respectively. We consider the impact of CRs accelerated by accretion shocks, and show that clouds with star formation efficiencies greater than 2% have X _CO = (2.5 ± 1) × 10²⁰ cm^-2(K km s^-1)^-1, consistent with Milky Way observations. We find that changing the CR ionization rate from external sources from the canonical ζ ≈ 10^-17 to ζ ≈ 10^-16 s^-1, which better represents observations in diffuse gas, reduces X _CO by 0.2 dex for clusters with surface densities below 3 g cm^-2. We show that embedded sources regulate X _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 _CO in extreme CR environments. X _CI has been proposed as an alternative to X _CO due to its brightness at high redshifts. The inclusion of internal CR sources leads to 1.2 dex dispersion in X _CI ranging from 2 × 10²⁰ < X _CI < 4 × 10²¹ cm^-2 (K km s^-1)^-1. We show that X _CI is highly sensitive to the underlying CR model.