Cosmos Seminar

Cosmic Ray Ionization Rates in Star-Forming Clouds from STARFORGE Simulations Compared with Observations

Cosmic rays (CRs) play a fundamental role in regulating star formation processes in molecular clouds, yet the cosmic-ray ionization rate (CRIR) remains difficult to measure directly and is typically inferred from observations of molecular ion tracers. Discrepancies exist between the CRIR derived from chemical tracers and direct measurements from the Voyager spacecraft, but it is unclear whether this tension arises from uncertainties in observations or reflects the true variations of the Galactic CR distribution. To determine whether this tension is physical or observational, we evaluate CRIR estimates derived from the CR tracer relations in the literature. We apply the time-dependent gas-grain chemical code UCLCHEM to simulation snapshots from STARFORGE (Fitz Axen et al. 2024), which explicitly models cosmic-ray transport in a 2000 M☉ molecular cloud under fiducial and high CR energy densities. By comparing the predicted CRIRs against the true values computed from simulations, we find that the simpler {H_3^+}-CRIR relation systematically underestimates the true CRIR by a factor of ~2, while the more complete relation recovers the true CRIR within <~5%. On the other hand, the high-CR-energy models produce CRIR values in closer agreement with observationally inferred values than the fiducial models. These results suggest that the {H_3^+}-CRIR relation can reliably recover the true CRIR when applying the known observational constraints, and that the observed discrepancy between indirect and direct CRIR measurements may reflect genuine variation in CR energy density across different cloud environments.