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 that includes cosmic-ray (CR) attenuation in situ to quantify the impact of different CR models on the CO-to-H 2 and CI-to-H 2 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) × 10 20 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 × 10 20  <  X CI  < 4 × 10 21 cm −2 (K km s −1 ) −1 . We show that X CI is highly sensitive to the underlying CR model.