# # Program information file # PROGRAM_ID 2021B075 PROGRAM_TITLE Cosmic-ray ionization rates inferred from H3+: testing predictions of cosmic-ray transport theories PROGRAM_INV1 Nick Indriolo PROGRAM_INV2 Alexei Ivlev PROGRAM_INV3 Kedron Silsbee PROGRAM_INV4 Paola Caselli PROGRAM_INV5 David Neufeld PROGRAM_SCICAT galactic/interstellar medium PROGRAM_ABSTRACT_BEG Interstellar chemistry is primarily driven by fast ion-molecule reactions. In dense and even diffuse clouds this reaction network begins with the ionization of H or H2 by cosmic rays, as ionizing UV photons do not penetrate such regions. This makes the cosmic-ray ionization rate [CRIR] a vital parameter for modeling the chemistry in a variety of regions, from diffuse atomic clouds, to cold molecular cores, to protoplanetary disks. As cosmic rays traverse the ISM they interact with ambient material and magnetic fields, resulting in energy losses and altered trajectories that change the CRIR. How much the ionization rate changes depends on the dominant regime of cosmic-ray transport [e.g., free streaming vs. diffusive scattering], and recent predictions from transport theories give the relationship between CRIR and N[H2]. By making precision measurements of the cosmic-ray ionization rate in diffuse molecular clouds, we will make the first determination of the cosmic-ray transport regime in this environment. The most reliable estimates of the cosmic-ray ionization rate come from observations of the molecule H3+, and these estimates have the lowest uncertainties when N[H2] has also been directly measured from UV absorption. As such, we are targeting the H3+ absorption lines near 3.7 microns in sight lines where N[H2] has already been measured, in order to make precision measurements of the CRIR. Typical H3+ absorption features are weak [<2%] and narrow [FWHM~5 km/s], requiring high spectral resolution and high S/N for detection. IRTF/iShell provides the opportunity to more than triple the current sample of diffuse cloud sight lines with precision cosmic-ray ionization rate measurements over both the fall and spring semesters. PROGRAM_ABSTRACT_END