Contributed Talk - Splinter ISM

Wednesday, 23 September 2020, 14:56   (virtual room F)

Investigating the Accuracy of Cosmic Ray Ionization Rate Calibrations

Brandt Gaches, Stella Offner, Thomas Bisbas
Universität zu Köln,University of Texas at Austin, Universität zu Köln

In well-shielded regions of molecular clouds, cosmic-rays play a dominant role in chemistry. The chemistry in these regions goes through ion-neutral reactions, controlled by the ionization fraction. As such, the cosmic ray ionization rate is one of the most fundamental parameters in any astrochemical calculation. Constraining the cosmic ray ionization rate in molecular clouds is hindered because the charged particles consisting the cosmic rays are not directly observable in these clouds. Therefore, there are numerous calibrations which have developed using simplified chemical assumptions to infer the ionization rate from molecular line observations. I will present the results of astrochemical calculations in which a spectrally-resolved treatment of cosmic ray attenuation is included in one-dimensional photo-dissociation region models. Since we include cosmic ray attenuation in-situ with chemistry, the models can better probe the accuracy of these calibrations. We focus on methods using H3+, HCO+, N2H+ and the carbon cycle. We find that i) when using column densities, the ratios of HCO+/N2H+, CO/C+ and C/C+ do not particularly trace the density-weighted cosmic ray ionization rate, ii) local approximations with H3+ and C/CO systematically under predict the ionization rate unless cosmic rays are the dominant source of ionization and iii) the local HCO+/N2H+ abundance ratio is rarely a unique tracer of the cosmic ray ionization rate. We conclude that astrochemical models including column-dependent treatments of the cosmic ray ionization rate are crucial to constraining the ionization rate from molecular observations.